Here's what you can do in GTA Online:
- Buy an apartment (Why?) -
Increase your skills (Shooting, driving, sprinting, lung capacity, etc)
- Buy/customize cars, clothing, tattoos, guns, etc
- Insure cars so you'll get them back even if destroyed.
- Buy and sell stocks. Sounds boring but a dev said that the things you do along with basic transactions will create changes in the market.
- Plan bank heists and mug shops with your crew.
- Do missions given by NPCs for cash and reputation.
- Mark yourself out of PvP. I guess this is for when you don't want to be messed with while shooping or doing missions. Still weird.