Goat Simulator is a short game, it doesn't take too long before you've explored practically everywhere, and done almost everything. Some of the stuff, however, you just have no idea exists unless you look at the wiki.
The mutators are great fun though, for example the Penguin/Grapple Goat/ DEADGOA7 plus a hat that you see above. Also the Giant goat:
And the Fast Goat:
You can also steal money...
...And then buy hats from this shop.
Blowing up cars is one of the most satisfying parts of the game
Golden goat statues (which unlock a mutator every 5 you collect I think) can be really hard to get to.