Well, it depends.
I basically goofed off with setting up Forestry for the bees, before setting up a Magma Crucible -> Industrial Centrifuge pipeline for copper and tin. Thaumcraft 3 is also a lot of fun, and even has a research aspect - as the minecraft forum post says, mine up a Crystal (Those weird glowy cracked rocks you'll sometimes find), and make an Apprentice Wand out of the crystal, a stick, and a gold nugget, then use it on a bookshelf to get the Thaumonomicon. A lot of really strange and neat effects in there.
You can also use the Crucible to get obsidian for a portal - you'll need blaze powders to set up a Blast Furnace, which then feeds into the hardmode IC2 stuff - namely the I. Blast Furnace. I. Electrolyzer is really useful, too.
Note to self: get around to making an OV detector, I need more diamonds!
You could, in principle, also visit the Twilight Forest, but I haven't gotten around to it.
Last, if you found a block of diamond (like, 9 diamonds) and a vein of silver somewhere, you could try setting up a Factorization line - among other things, it gives a silktouch tool and a way to get 2.25 diamonds per ore, without messing about with GT machine casings - but that's for later on.
Edit - ninja'd on the blaze powders. Yeah, I forgot about the induction smelter. Rich slag has a few very strange behaviours that make it very powerful. If you're working with GT though, you might want to pulverize your iron ores instead - ferrous dust = nickel, and you'll need some for later. Gold, on the other hand, has no pulverizer bonuses, so smelt that directly.
For added hilarity, you could try messing with Soul Shards to get even more blazes. Blaze powder is very useful, even after you set up the Blast Furnace.
