I like observation towers, especially if they're cheap, because I love to sketch from them.
The ones I've been to, starting with my favorites (these are all my pics/sketches):
Coit Tower, San Francisco:
Torre Asinelli, Bologna: If you don't know this one, look it up.
Tower of the Americas, San Antonio (incidentally taller than the Space Needle
Duomo San Pietro, the Vatican:
Campanile San Marco:
Top of the Rock:
Empire State Bldg:
Sky Lobby of Chase Tower, Houston (free!):
Griffith Park Observatory, Los Angeles: