Usually your u/s tech will be pretty forthcoming about the important things your baby is doing or has achieved. I would ask how much the weigh and how big they are, one of the neat things the tech told us at our last ultrasound was that baby A was practicing his breathing which meant he was oxygenating well and something else (can't remember right now). She didn't mention anything about if baby B was doing it and I forgot to ask... I know with my son at a later ultrasound they pointed out little white flakes in the amniotic fluid and said that was a sign of lung maturity (he had been practicing his breathing as well) so if you want you could always ask about those things or at what time they would expect to see those things. Other than that I can't be much help I guess, try to relax and enjoy it!!!!
I would ask what bub weighs. I love knowing exactly how big my baby is!