One advantage of Goat Simulator over a pure physics sandbox is that it gives you some direction in the form of "quests" tasks of varying complexity and obscurity that you can do if you're so inclined. The primary reward of completing quests is that they are crossed off your list of uncompleted quests (which is, admittedly, not nothing). Certain weirder or more difficult quests will give you new skins and (largely counterproductive) special abilities, but there's no way to tell which ones will do so beforehand.
Following a guide, I've completed all but one of the quests. The only one I have left is from the "MMO Simulator" - I have to lick 10,000 items. Clearly a joke quest, and though it bugs me that it remains uncompleted, I'm not inclined to just sit obsessively mashing the tongue button thousands of times for no particular reward (or even a really cool reward). There's not much left to do after this, but luckily I still have a bunch of Achievements to purse.
I think playing Goat Simulator for 20 hour straight is not the optimal way to do things. I think the game works better in small bursts. You do one or two crazy things and then move on to something else. Trying to find new ways to waste time taxes my creativity after about an hour. Once I run out of easily attainable Achievements, I'll probably just wind up running around in circles until the clock runs out. Hopefully that won't happen too soon.