Most of them, but the companion quests are specially entertaining(Boone's and Rex's being my personal favorites). "That Lucky Old Sun" seems to be a favorite between players too.
I gave myself the side quest "Find all the unique variant of weapons". Might entertain you too. Sure is challenging sometimes.
Trying to find all the unique weapons is a great way to spend time. Another thing I personally like to do is fight in The Thorn, but beware, it can get rather glitchy down there. Visiting the Vaults late at night is also fun, Vault 22 especially.
Try also find all weapons and armor (including clothing and unique variants) AND having them at 100% condition.
You may also want to do some hunting, if you're low level, the area surrouding Jacobstown it's a good place to start (avoiding the Nightstalker's cave and hunting in broad daylight to avoid them), but once you reach higher level you might want to give yourself a challenge, like hunting Deathclaws on the Long 15, but, in my opinion, the best hunt would the Zion Park's (Honest Hearts DLC) Yao Guais, not only for the hunt, but also for the hunting grounds, Zion, beign a national park, has a lot o tress a big river, wich makes you feel just like a Pre-War hunter, if you overlook the fact that your hunt is a mutated form of a Big Game Hunt, that your threats are ants even bigger than the bears, Geckos, and heavily armed religious tribes, of course