Depends how far into it you're going to go. If you're going to disassemble the FCG, a punch is pretty handy. For anything serious, a vice block is virtually necessary. Want to build your own upper, a barrel nut wrench is crucial. To drive out the pin on some FSBs, you'll need a punch, and a pretty small punch to drive out the pin on the gas tube roll pin or trigger guard roll pin.
If you don't plan on any of that, a box of Q-Tips and your favorite oil is all you'll really likely need.