we did under the sea this year...
we made shells and fish (cut templates out of cereal boxes - lots of cereal boxes - and lots of different shaped fish/ shells) and decorate with whatever you like - we had painted/ coloured fish and shells, but also collaged with different coloured foils/ fabrics/ paper.
unroll a roll of plain blue wallpaper along the hall floor/ wall, and paint the seabed at the bottom.
add fish, shells, rocks, seaweed (green string etc) etc (made as above) as you see fit/ however many get made.
depending on how long you have or how ambitious you are feeling, you might choose to also do one 'main' focus, such as a submarine, or octopus, or diver etc.
you can also make fish out of empty soft drink bottles which can be covered in tissue paper and glue (like rainbow fish lol) and suspended by cotton from the ceiling to hang in front of the mural...
the sky is the limit really - most activities can be adapted for an sn audience.
have you got plenty of helpers lol?