Ringworld

The quick summary: a small group of aliens, FTL travel, a giant structure that wraps entirely around a star, and the core of the galaxy exploding. Unlike a lot of current fiction, it's a relatively short and fast read (no 1000 page monolith here). It has some nice humor elements mixed in, but more as a compliment than anything. A fun read that doesn't feel dated, which is not always the case for old scifi.