Make sure she gets the Amazon Appstore - they have a free paid app of the day. Most of the time it is a game but sometimes they have functional apps, some of them really useful.

I also like:
3G Watchdog - monitors our data usage
Award Wallet
Battery Time Lite - shows battery remaining as a percentage
ESPN Score Center
Evernote - super note taking app
Flight View - tracks flights real time
Flyer Talk
Gas Buddy - finds closest gas stations and show prices
Grocery IQ - grocery shopping list app
Kayak
Lookout - virus scanner and lost phone finder
Google Reader - RSS reader for all the cool travel blogs
Open Table - restaurant reservations on the go
RN Dining - shows Reward Network dining places
ShopSavvy - scan a barcode and it compares prices
Sky Scanner
Wi Fi Finder - scans for wi fi
Yelp

A lot of apps are hit-and-miss, you can always remove it if it is not worthwhile. Have fun exploring!