I'd say a bone folder, good scissors, good adhesives (liquid and dry), a cork board and piercing tool, a package of mixed color brads, a clear ruler (love my CS one!!), some envelopes, a couple of those decorative paper mat stacks that are photo mat sized (easy for layering on cards and great for quick cards), a pack or 2 of mixed colors of 8.5 X 11 cardstock... and then from there maybe decide whether you want an embellishment side or a stamp side to add to it. (I'm thinking in CS pb terms now! lol!)
Embellishment side: fibers, ribbon, brads, eyelets, flowers, metals... whatever you think would give her a good start of stuff to dabble with. Stickers? Glitter? Glitter glue? Fun flock?
Stamp side: A good basic black pad is always a good choice, along with either a bottle of stamp cleaner or a travel size pack of baby wipes (lanolin free!), and then maybe some $1 stamps sets and an acrylic block. If you have the money to buy an embossing gun, then I'd also get a Versamark pad and clear, gold, and black embossing powders. (I have 5 drawers of embossing powders and these are the ones I always seem to go back to.) Plus, she can explore the fun of watermarking and using chalks with the Versamark pad.
Also, if you live near a Michael's or Joann's, or any other store like that, sign her up to receive e-mail coupons so she can splurge on new tools at a sale price. And set up play dates to have her use some of your tools so she can try them to see if she really wants to spend the money for her own. When I first got into stamping, one of my best friends was the enabler and she and I would go shopping and buy different things we both wanted to try, thus splitting the cost while getting to play with new toys. This is also how I collected so many embossing powders... every time she or I bought a new one, we'd share! I can buy the little jars for $.30 each at a local craft store, so it works out quite well.
My $.02 worth...
Monica