Dropbox--access important files and documents (Videos, PDFs, etc) from the cloud. Service is free up to 2 gigs. Great for PCs as well.
Air Video-Stream video from your PC to your iPad. Great for bedtime or couch watching, while you're playing games. LOL.
Netflix--Netflix instant Q access on your iPad.
Instapaper--You need to add the bookmarklet to your browser, but it allows to read longer form articles at a later time and accessible by another device, like a smartphone or iPad. It strips the ads and formatting, so you only get the essential text. Great product if you like doing a lot of long form reading from online sources.
Flipboard--A sort of RSS aggregator that present your feeds in a very tablet friendly manner. You can also add Facebook and Twitter feeds. Great IU design
The Apple apps are also great--Keynote for presentations, the new Garageband and iMovie--if you have the iPad 2--and Facetime.
Kindle app if she's an avid reader.
Games: Infinity Blade, World of Goo, Cave Shooters, Angry Birds , Rage HD.