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.
1. If I were u I would build some sort of defence force. With numbers of different cannons like tnt, sand and player launcher along with tamed wolves and anything else that classes as a weapon.
2. Automatic wheat farm, I have a duel wheat farm on my world and it works really well.
3. Wheat and sugar cane along side rivers, it looks really good and I recomend a scarecrow made of an armour stand wearing a pumpkin and leather armour.
4. Make a mining hub, add chests, furnaces, beds and crafting tables and a Minecart track leading down into the mines, if u use storage minecarts to transport your items and just go back mining it will save u time. Make it at about level 12 ( diamond level). That is were I made mine and my automatics farm. Also raid an ocean monument, it is really fun and and easy way to get fish, sponge, all three prismarine blocks shards and crystals and sea lanterns.
hope I help and I have build all of these on my Xbox one smp and they all work great and have a nice day!