Reddit mentions of microtivity IL188 5mm Assorted Clear LED w/Resistors (8 Colors, Pack of 80)

It really depends on what your end goal is.

If you want to stick with Atmel, then buy an Arduino or Arduino clone. This will cost you ~$40 or so depending on which model you decide on. With an Arduino, you can use the Arduino libraries, or write low level code (I suggest at least trying low level code). If you decide you want to build projects with stand alone AVR's, then it will be a good idea to purchase a flash programmer as well ($10 to $20). The chips that come with an Arduino contain special code that lets you load new firmware over a serial link (called a bootloader). Raw chips lack this, so if you want to program a bare chip, you need the tool. You can even just use the tool to flash the arduino bootloader onto a bare chip.

If you aren't sold on Atmel, TI offers a very competitive option in the MSP430 Launchpad ($10). There is a port of the arduino code and IDE for this board (Energia), so you can still get the arduino experience if you wish. Otherwise, learning to program MSP430's at the low level really isn't that tough once you get a feel for it. Besides, the cost, a big advantage TI's offering has is that the dev board contains a built in programmer, which can be used on external chips. When you move to a stand alone chip in a project, you can program the bare chip by running 4 jumper wires from the dev board to the project....so there is no extra programmer to purchase. If you were to replace the ATTiny in the project with a MSP430G2210, you could accomplish the same thing and program it with the launchpad

Once you have a dev board, I would pick up a solderless breadboard and jumper wires (less than $10 on Amazon). Based on your project idea, I would also either buy a small resistor pack (~$10) and some LED's, or something like this. That would get you started towards building your project on a breadboard.

Just find a nice supplier of LED bulbs, resistors, and battery cases. My cases came from RadioShack and were about $1.50 each, and you can get a bulk pack of 80 LEDs and resistors off Amazon for $9. There are very few blasters that can't fit an LED into their front ends somewhere, and as long as there's enough space in the blaster for your batteries, the wiring real estate is almost negligible.

Of course, it requires a lot less tinkering if you can tear down an existing flashlight and fit it in the case, but where's the fun in that? :P

Edit: Also, that Rayven is beautiful. I need to get myself a Rayven.

I've decided to mod the stock LED light that comes with the Fluval Spec V. I know I'm going to need to upgrade to something with more power eventually, but in the meantime I wanted to do some experimenting. I ordered a color assortment of 5mm LED's from amazon for like $2 and I plan on replacing some of the daylight white LED's with a mixture of blue and Violet. I want to see if a low light coral can live under the stock light with this mod. I'm hoping that the blue and violet LED's will provide a better spectrum of light for the coral to photosynthesize even if the wattage output is crap. I believe these 5mm LED's are .07 watts each. There are 37 LED's total -27 on the daylight switch and 10 on the moonlight switch. I'm interested to see what combination will look best. I think I will start with 12 White, 8 blue, 7 violet. Or maybe 14W, 7B, 6V. I will be adding pictures of the process and final result as soon as the LED's arrive.

TL;DR I thought it would be interesting to see what effect I can produce with the stock light with some cheap LED's and a test of my soldering skills before I decide to buy a retrofit kit with 3 watt LED's that cost $70+

Honestly, i don't like it. Here's why: all you customize is bunch of leds, you can just buy a goody bag(http://www.amazon.com/microtivity-IL188-Assorted-Resistors-Colors/dp/B0060FGA8A/ (comes with tons of resistors, more than you need) from amazon and that makes your entire thing pointless. Also it's damned pricey.

If you included a real selection list of things like different sensors(acceleromters, gyros, magnetometers, preassure, level, light, sound), different actuators(motors, servos, etc..), assortment of indicators(led, ir, 7 segment, led matrix/grid, lcd screen), shields(usb host, xbee, motor drive, relay), ic packets(regulators, logic gates, op amps, shift registers, etc...) and stuff like that at reasonable prices then I could see it being something great, but otherwise i just think it's pointless.

Well, breadboards run pretty cheap... And size matters here. Sometimes a smaller board is better, but a large board gives you more room to work with. So I'd recommend getting both! A small board will be nice for smaller projects, but larger boards will be nice for more in-depth prototyping that require more room to work with. And he can always use a small area of a large board when he doesn't need it all... but a large board won't fit into a small boat or airplane if that's what he is doing... So, at a good price, both can be beneficial.

Other than that, he'll need jumpers (male/female), and maybe a ribbon cable. Ribbon cables need a T-board (or equivalent) to connect to the breadboard with, but jumpers just plug into the GPIO pins. With a small breadboard, jumpers are favorable, as they only take up as much space as one pinhole, and they come in many different colors and lengths. A larger board can accommodate a ribbon cable and T-board more readily... a T-board can take up a LOT of space on a small breadboard REALLY quickly.

Word to the wise: if you buy a ribbon cable and T-board, you are going to want to know what type of Pi he has. The original versions had fewer GPIO pins than the B+ and the A+ versions. This means that a larger ribbon cable wouldn't work with the original Pi's. If he has an A or B, a 40 pin cable/T-board wouldn't work as they're designed for the newer B+ and A+. If you need help identifying which board he has, we can help you with a photo or a description of the board...

Also, a resistor kit and a capacitor kit would be good to have, if he doesn't already have them. Most projects require that you use resistors/capacitors somewhere, and the kits usually come with several of the most used resistors/capacitors, which should tie him over for some time.

Then you should get him an LED set. LEDs are nice because they can be used as a "proof of concept" tool. Since everything into and out of the Pi is digital, all you are doing is giving an item voltage. So it doesn't matter if you're using a buzzer, motor, servo, or LED... they will all work interchangeably. He can use an LED to prove that his code and wiring works when the light comes on... then, when it all works, he can swap the LED for a buzzer or something, and everything should still work, just with sound instead of light.

You could also buy him some sort of case, but these aren't always necessary. I don't use on on my Pi, and it works just fine... I just have to make sure I'm not shorting it on anything, or breaking it in some stupid way. These vary so much in design and price, I'll let you search for them instead of providing a link... just remember, like the ribbon cable, cases will obviously depend on the model he has.

Personally, I would stay away from potentiometers and photoresistors since the Pi only has digital inputs. Both of these items would need to be attached as analog items to work properly, and that means buying extra boards for correct functionality. The Raspberry Pi can not natively read analog inputs, so these items would only be frustrating to him since he can't use them without extra parts.

This should get him started in coding and prototyping. If he's looking for motors, servos, or speakers, you can get him those too, but they're not necessary to tinker with a Pi.

There are also kits that offer these items, but they're usually more expensive than they have to be, and they may or may not have everything you're looking for. Also, there are other places to shop for these types of things, but I use Amazon because it was the quickest and easiest place to search for examples of all of these things for this post. If you're smart with your shopping, you can get all of these items for ~$100, and might be able to have them shipped before Christmas... but you're going to have to hurry.

EDIT: Added links since you said you knew "nothing" about these items. This should make searching easier since you can actually see what I'm talking about...