Pretty much everything I use my 13" MBP for now... plus all the apps I use on my iPhone. The iPhone will get many more hours of standby vs use as every app I use on my iPhone will be that much easier for me to use, read, or play games on the iPad.
Movies, email, surfing, pictures, gaming, reading emags/ebooks, listing to music etc...
I rarely use my iPhone to surf unless I have no other alternative but find it a pain to to deal with all the zooming/panning to get around a website. I've done some reading on the Amazon app but again, spend more time flipping pages then reading, never felt right or enjoyed it to get through a book on such a small screen. Look forward to a more "normal" page layout and ability to read articles without moving the page ever few sentences.