Industrial craft is fun.
But I'd recommend any number of mods that include a new dimension or bosses, those are the best in my opinion because they add so much to do to the vanilla game.
That space mod where you can go to the moon and mars and that one place with the monoliths of which I very much do not like at all is very fun. Seriously, I get sweaty as all forget in the monolith place, I don't know why.