If you are gonna be at the Con, try and find a hotel close enough to walk. Parking sucks (really badly) and you will not enjoy trying to find a spot every morning.
Also, note the prices on passes. Usually, 2 days is more expensive than just getting a full weekend pass. If you're gonna be there more than one day, it will probably be cheaper to just get full passes. Also, book your hotel ASAP. Many are full already.
If she is into games of any kind (or fantasy, or pretty much anything like semi-dorky), there will be tons to do. The con itself is just rooms upon rooms of every type of game you can think of, from CCGs to board games and lots more. I've never been into it but there are probably panels with artists and designers and such, demos of anything and everything you can try, and lots of weird, dressed up people to stare at (TONS of these). Again, magic is only a small part of the con (one room really, and often a shared room with other stuff). It is one of the biggest Cons in the country and if you are the type who enjoys those things, Gencon will be great.
My opinion of Indy is that it kinda sucks. I can't recommend much else to do there sadly.
Ditto. It may be a bit expensive, but stay at a hotel close to the con, LHC said. We made the mistake of staying to far away and the Taxi costs were stupid expensive. And Indy, as a town, does suck. In fact, I would go so far as to say to say it's a shit hole. Gencon might be the only thing that town has...