I recently reread Time Enough for Love and it still holds up -- it's not nearly as anti-feminist as some of his hard sci-fi. Stranger in a Strange Land is slightly less relevant, though a great deal more subversive and slightly more anti-feminist.
On second thought, I would give neither book to e.g. my 13-year-old cousin.
I probably should have mentioned that I was a teenager in the sixties, and what was available to me was the juveniles. I think my favorites were The Star Beast and Have Spacesuit, Will Travel, with Citizen of the Galaxy trailing not too far behind.
I think I only read Rocketship Galileo once, even though Nazis in abandoned alien tunnels on the moon seemed wonderfully extravagant. I reread it recently, and found it had so much immediately-post-WWII material as to be rather sad and grim compared to most of the juveniles.
I bought my niece a Kindle that just arrived and I'm about to load it up with books to give it to her tomorrow for her birthday. I've decided to be a sneaky uncle and include good books that can teach better abilities to think or at least to consider science cool and interesting. She is currently in the 4th Grade with 5th coming after the Summer.
She reads basically at her own grade level so while I'm open to stuffing the Kindle with books to be read when she's ready, I'd like to focus on giving her books she can read now. Ender's Game will be on there most likely. Game of Thrones will not.
What books would you give a youngling? Her interests currently trend toward the young mystery section, Hardy Boys and the like, but in my experience she is very open to trying new books with particular interest in YA fantasy but not much interest in Sci Fi (if I'm doing any other optimizing this year, I'll try to change her opinion on Sci Fi).