T Flip-Flops for pistons repressing a fall of sand, not too tricky theoretically.
Also, I've built a secret room thing under a fountain on some island. I have no idea if it's any use, but if anyone builds in there, careful, because the water'll wreck the circuit if it gets let into the wrong places.
Thanks for the heads up. No pressure...BUT EVERYONE WILL DIE IF YOU LET WATER IN