Quiet things you can do at 3am...

Iron.
Knit.
Read on another level of the house.
Type - with same caveat above.
SLEEP - this is my preferred activity at said time.
Empty/load dishwasher using extra precaution for maximum noise reduction.
Laundry (ours is in the basement)
Hand wash floors - this is NO FUN
Watch tv - but there's nothing on, old dvds would be good here - except the funny ones, because then you wake everyone up from laughing too loud....
Shovel snow
Think about things you can do when everyone is awake, when you'll be ready to sleep
Plan the ultimate dream vacation
Plan a vegetable garden, or even a flower garden - these things take research!
Surf the net
that's all I got. I'm actually TIRED!!