Sorry for my late response, totally forgot about this thread!
If you're in for the long-haul, I would look at The Hyperion Cantos by Dan Simmons, Revelation Space by Alistair Reynolds (or House of Suns if you want a stand alone novel), or the Xeelee Sequence by Stephen Baxter. Really far-reaching hard science-fiction. Will blow your mind!
If you want a short mind-fuck Tau Zero is a great short novel. If you want to wonder about aliens then try The Mote in God's Eye or Rendevous with Rama.
The Forge of God's quite good if you want a take on alien invasion.
And The Stars My Destination is a classic, Gully Foyle is one of the great anti-heroes in literature.
I don't think you can go wrong with any of those.
Happy to help you more if you need it, it's my genre!