Last fall I offered a couple of weeks' intro to sf in an Introduction to Literature course. A student just asked for some summer reading suggestions. I'll bet my recommendations will differ from what most of y'all would send, but I was thinking of ones that I could personally attest to finding to be of special interest.
I've read C. S. Lewis's science fiction trilogy about a dozen times now: 1) Out of the Silent Planet, (2) Perelandra, (3) That Hideous Strength. They get my highest recommendation. After you read these, you might trythe book that Lewis said inspired him to write sf, Lindsay's A Voyage toArcturus. Our library has a copy with a hideous cover that was probablynot even painted to illustrate the book, so it has nothing to do with thecontents. Google Images will show you some better covers! But thetext in our copy should be fine.
H. G. Wells is the father of sf and his Time Machine and War of the Worldsremain very readable, also short stories such as "The Crystal Egg,""The Sea-Raiders," etc.
George R. Stewart's Earth Abides is a classic in the post-disaster genre ofsf. I've found Wilson Tucker's The Year of the Quiet Sun an impressivenear-future time travel story.
Walter M. Miller, Jr. wrote the famous sf novel A Canticle for Leibowitz. He wrote some excellent shorter fiction too, including Dark Benediction and theheart-breaking Conditionally Human.
In 220 we read Algis Budrys's Rogue Moon. You could try his sort novelWho?, which is kind of science fcition meshed with Cold War thriller.
When I taught American Lit a few years ago, I included Ursula Le Guin's TheLathe of Heaven.
Not really sf, but an imaginative and enjoyable mash-up of sf, weird fantasy,and thriller, is Tim Powers's Declare.
I thought Robert Charles Wilson's The Chronoliths, from a few years ago, wasvery entertaining, also Blind Lake and Spin.
The most recent sf novel I have read is Weir's The Martian, published thisyear. Very techy, but impressive.