I have two ideas.

Well i was on my server and i decided that i wanted to fight people on unfair odds, so i built i giant arena where battles would be hosted. since my server hasn't gone up yet i can build another arena like mine on FlatLand. Also I didn't try this idea but i was thinking about building kind of a small city of shrine or something big, but contained in a glass sphere underwater. That way to get inside you would have to swim down.