First off Congratulations on bring the babies home.
We decided to bring in a card and a pie (from a local bakery) for the NICU Staff. I know it wasn't much, but I think they really appreciated the card.
I am a nurse at a ped hospital. I card with a nice note is the best gift ever. I save my cards and read them after a really sad day at work. The nice letters help me realize that the work I do is important and appreciated- even if the outcome is not what anybody wanted.
We often receive goodies to eat, which we share with other staff, and sometimes receive flowers. We don't expect a gift- I need to stress this. It truely is a rare occurance, and a nice surprise. Another nice surprise is when we see them later, after they have grown up some. So maybe down the road, a card with a pic will make them smile. My good friend works in the NICU (for 25 years) and can remember most babies she has cared for. Some parents still hunt her down for a visit, if they come for a clinic appointment at the hospital.
I do not work in NICU, but often care for them later. I have worked in the emergency room and recovery room and often see the tiny miracles come through. Congrats on your babies!