What should i do in vanilla minecraft?

Personally I like doing stuff with redstone and command blocks but that may not be your type of thing. What you can do is download challenge or Adventure maps. Here's a link talking about how to download them:

http://minecraft.gamepedia.com/...ustom_maps

BURN SHYT

Try going on the forums and doing one of the Challenges, especially the survival building challenges. I recommend Fantasy City Challenge. Or go on a server (Mineplex is really fun).

Build a castle