We did a New Years cruise with our kids on Royal Caribbean years ago and it was the best cruise we ever took. There were parties all over the ship and everyone had a great time. We are booked again this year for New Years on Celebrity, I hope it lives up to our past experiences.
If you want to gift the crew, cash is the preferred option by far. Cabin decorations are always fun, but the lights might attract too much attention from mischievous individuals.