Some titles:
Dune (Herbert)- great as a stand alone or as the portal into a huge (sometimes messy, ymmv) world of books
Glory Road (Heinlein)- Mix fantasy and sci-fi? Sure! And it’s fun, too
The Mote In God’s Eye (Niven/Pournelle)- fascinating imagination of what a totally alien culture might be like. Plus, it has big spaceships. The individual sub stories were well done, I thought
Ringworld (Niven) - As a stand alone or as your own intro to more Known Space stories. Excellent visualizations.
The City and the Stars (Clarke)- one of my faves as a youth, reread it a couple of years back, still good. A better version of his earlier Against the Fall of Night. Set so far in the future, even we are somewhat alien (in thinking, at least). Intelligent machines, space travel, mystery, etc …
Up The Line (Silverberg)- a different take on time travel. campy, weird, fun
The End of Eternity (Asimov)- another different look at time travel (of course you might also want to read his lesser known work, the Foundation series)
A Princess of Mars (Burroughs)- may be campy, prejudiced, silly, what have you, but boy is it fun. Inspired a lot of early sci-fi writers and even some who grew up to be scientists.
The Time Machine (Wells)- time travel? really? Sometimes, a story is a classic for good reasons. This is one of those times. has been reprinted in paperback many times.