Reddit mentions of Sargent Art Classic Fine Tip Markers in a Case, Set of 30 and Dover Creative Haven Snowflake Mandalas Coloring Book by Marty Noble (Bundle of 2)

Well, everyone has different preferences, but I'd suggest getting one with pictures (this is lovely and very popular - she might find her friends have got it also, if they color) and one with patterns, (don't have this one personally, picked it at random) so she can decide which she likes to color more.

Fine tip markers like Staedtler, and a set of decent but not top-of-the-line pencils like Prismacolor would be a good start. If that's more than you wanted to spend, a smaller selection would probably be fine, or a cheaper good brand such as Staedtler Noris colors which I personally love. Oh, and get her a pencil sharpener and eraser as stocking fillers :)

[edit for links] - not linking the Staedtler Noris colour pencils because I'm in the UK and am not sure the Amazon.com ones are exactly the same.