It's basically my favorite holiday, and my for-sure favorite "not-store" holiday.*
*Extremely basic way of saying "non-commercialized." I make the distinction because I also vehemently love Halloween, and I also count my birthday as a personal holiday. That's the top-three, folks. Shout-out to May the Fourth, and Talk Like a Pirate Day.
Dragons Magazine has a long tradition of April Fools articles (a favorite of mine being Flaws for Commoners, Dragon #330). The most recent issue of Dragon+ had good inspiration for me as well. While not specifically intended for April Fool's -- the recent article on the artist Tahra's cat-people images, Herding Tahra's Cats, served quite well.
How It Played with My Group: Having solved the mystery of Secomber, and captured the fleeing criminal for a bounty, the group travelled north to Longsaddle. After a day of shopping and gambling, they stumbled into an area where a Harpell wizard's spell component bag was attacked by her pet cat. The result being an out-of-body plane shift, into the demiplane of the Cat Lord, and into the bodies of existing cat people.
APRIL FOOL'S, SCHMUCKS!
What they didn't know immediately: the cat people were now in their bodies. They'd remain in a playful and confused static state, safely at the Wayfarer Academy chasing rabbits and mice, unless a PC did something... like get them killed. The more aberrant behavior was from a PC, the more aberrant their cat friend would be as well. The spell would automatically wear off in less than 24 hours, and they'd be back home in their own respective skins.
So, my players filled the night with cat puns... doing their best to play their borrowed roles. They fought a pirate rat king, and his evil wizard cat associate. They bought catnip off some sketchy hamster brothers. They battled temptation when mimics arrived looking like cardboard boxes. They defended the king, and were heartily rewarded -- and that's when the spell wore off and the cat souls returned home to find themselves being honored in the royal court (much to their surprise).
If you want to also run a one-off night on the Isle of Catlantis, please enjoy the following things I've made (for use with traditional tabletops, or Virtual TableTop like Roll20). Some are lifted from the article, some are original content, and some are reworked stuff I found on Google Images.