a boat, tnt cannons, towers/fort, piston inventions(elevators, secret doors, etc), and prepare for the ender dragon. hope this helped you out
Useful things: Automatic Wheat, Sugar cane, Melon, and pumkin farms, Trash Bin, Mob Spawner, secret passage & a trainstation.
Fun Things: Garden, Fountain, Lightpost, Pixel Art, A giant hole that goes to bed rock
A cool thing to build would be a lake. You can fish in it. You can make a fishing dock on it or just a dock in general. Those are pretty cool. Another good idea that's not for your backyard is some secret passages under your land. It could be some quick passages or just some cool tunnels.