Since celebrations don't seem to accomplish anything, the best happiness strategy I can find is to try to keep all cities just above 72 so they won't revolt.
At Normal difficulty, this means that for most of the game your people are underfed, overworked, and underpaid, and you still don't need to buid most of the happiness-enhancing improvements if you get Ramayana.
Yeah, it seems people are really content at the normal/easy levels. But try this in late game, with democracy, and on a hard level and it may backfire.
I don't know yet, because I haven't tested it myself!