As someone else said, boxer shorts or some other easy pull-on shorts and a pair of sweatpants. Make sure they're loose enough in the legs for your catheter tube to string out the bottom.
A couple T-shirts that are a little large so the doctors and nurses can easily pull them aside to get at the healing incisions.
Both of those are real important. The sooner you can ditch the gown, the more like a real person you'll feel. And you'll be much more comfortable moving around if you're not worried about your backside hanging out.
A pair of easy-on slippers so you can start walking in your unit before you can really bend over.
Easy-on shoes - no buckles, no laces - that you can slip into while standing. Again - so you can move around before you can really bend.
Portable DVD player and iPod (or other MP3 player) really help. You will hit a point where you feel much better and you're just sitting there waiting to pass gas or have a bowel movement before the docs will let you leave. You'll be bored.
The music on headphones also made a big difference for me. If you can, get a couple albums that you can easily lose yourself in and almost be hypnotized or lulled by. Sleeping can be hard the first couple days and something that can make your mind drift along really makes it easier.