These are not necessarily "new" ideas, but here is what some of the teachers are doing at my school for next year.
Under the Sea (Mine)--I have fish hanging from the ceiling and am decorating with lots of blue!
Read, White and Blue
One teacher decorates with a nature theme using items she has found over time in the woods/outside
Crabs/Small ocean animals
SC History (We have a teacher who teaches SS and decorates her room with SC items since that is the main focus for the year.)
I hope this helped in some way. Maybe it will at least get you thinking a bit.