Go Swimming
Swim in the sea! You´ll already be wet so the rain won´t matter.

Visit a Cenote
Swim in a cenote. Same as above: you´ll already be wet so the rain won´t matter. If it's a chilly day, keep in mind that cenote water is a bit cooler than the sea. But if it's a hot summer day, this is a great idea.

Have a Massage
Most Tulum spas have indoor massage rooms, so you won´t get soaked. Some spas offer in-room massages.

Take a Yoga Class
Many hotels in Tulum offer yoga classes. Check out the schedules and do some indoor yoga.

Have a Drink
There are several bars along the beach with palapa roofs where you can have a drink and hang out.

Get a Mayan Astrology Reading
Some spas along the beach can set up a session for a Mayan astrology reading, which can be a fun and unique experience!

Experience a Temazcal Ceremony
The temazcal is an indigenous sweatlodge. It's indoors, so you won't get wet until you come out, and it's tradition to take a shower afterwards anyway!

Experience a Floatation Tank
Some spas have floatation tanks, and since they're indoors, it's a great place to relax and not get soaked in the rain.

Tips before you go:

If you know you'll be traveling during the rainy season, bring a deck of cards, a couple of great books, and maybe some travel board games with you.