Lem's Eden was one of my favourites as a kid and works on many levels:

• Cool action and adventuring on a mysterious alien planet.
• Plenty of gadgetry of both human and alien origin. Including some of the earliest description of nanotechnology.
• Social commentary about interfering with an another civilization, oppression, totalitarianism and control of information.

Niven's Known Space series, especially short stories like Neutron Star.

Clifford D. Simak has two fabulous stories, They Walked Like Men and The Goblin Reservation which have the perfect mix of action, humour and societal commentary. It's hard to beat the latters main cast consisting of a university professor searching for dragons, a neanterthal man, a girl with a pet sabertooth tiger and a ghost with amnesia. Hilarity is quaranteed to ensue.