What megastructures should I build? Any tips to make it more cooler?

[Vin__]

I've built a couple, the STL Copernicus (A doubled O'neil Cylinder colony ship), the donut kerbin, and a couple more like an artificial planet and the Halo itself from the game Halo. I'm not so sure what to build though, please provide ideas, I'm really bored. But I would also like some tips too. Like, how to make a hollow sphere with procedural parts? Or are there mods that could help?

I'd be happy for your help guys. Thank you.

[Vanamonde]

Make a ring around an asteroid.