Not any redstone stuff please just builds

Or you can create a castle or an enormous city or the international space station

You could build a library

How about a recreation of an area from another game, like the Judgment Hall from Undertale, or the Great Hyrule Forest from Legend of Zelda?

Underwater base connected to your house to use it to hide from your friends?

School

Build a university campus
I did it and it was really fun

I will like the ones I shall do