Kendall Jenner Stuns in Bikini, Soaks Up Sun on Spanish Vacation

Sun's out, buns out! Kendall Jenner's soaking in every last second of summer, showing off her supermodel assets ... getting her tan on while vacationing -- and, hopefully, she applied sunscreen liberally -- 'cause her thong clearly doesn't provide much protection.

The cheeky model and reality star graced the beach in Ibiza ... hitting the water on all manner of aquatic vehicles -- boat, jet skis, you name it. If it floats, KJ's taking it for a ride.

KJ let close pal Fai Khadra drive during the afternoon fun -- the one time Jenner covered up just a bit, going safety-conscious in her svelte life jacket.

KJ dropped it low for a few fun photos on the ship's bow too ... laughing while hanging out in the shade of the coastal shelf.

BTW, looks like Kendall's beach day wasn't a one-off -- 'cause she posted a couple more photos in a different-colored bikini.

Just Kendall and friends in Ibiza this week it looks like ... with no whiff of her on-again, off-again flame Bad Bunny.

Remember, the two hit the Mediterranean back in June with Jeff Bezos and Lauren Sanchez ... keeping a pretty low profile while grabbing dinner in Greece.

KJ and BB have rekindled their flame in recent months -- after initially breaking up late last year, they've been traveling together again, though less frequently than when their romance first raged.