A small trampoline where you are flung up at the push of a button.
This trampoline is made of slime blocks with a sticky plunger in the middle that is extended by the button. This is what it looks like without slime blocks:
As you can see in the picture, there are sea lanterns in the bottom. These are there only for lighting and can also be replaced by Glowstone.
This is what the redstone pipeline looks like:
I covered the trampoline with stone steps so that you can't look under the trampoline and so that you can also walk on the trampoline.