If you want to take a dip, there's plenty of places to do it. In Riga, Lucavsala is an option, as is Ķengarags, Bābelītis, Ķīšezers, which can all be reached easily with public transportation and a little walking.
For a real outdoorsy feeling the Bābelītis lake in Jugla is a very nice option as it's surrounded by forests. It has picnic tables, and even a dedicated municipal officer making endless rounds across the tiny lake with a rowboat.
While if you want to look at the Riga skyline while sunbathing there's Ķīpsala right by the Vanšu bridge.
Many of the swimming places come with beach volleyball courts, ping-pong tables and more.
Here's a list of Latvia's official swimming spots that lists safe-to-swim areas in Rīga and across Latvia.
The active life is worth living
Want to get moving? Rollerskate around the Mežaparks neighborhood, play beach volley with your friends in the Lucavsala public courts (both in Riga), or hop off to Sigulda to make your way through the 82 obstacles of the Mežakaķis amusement park.
Music by the sea
This weekend also has the Summer Sound festival in Liepāja, Latvia's third-largest city. Leaning slightly towards electronica, the annual shindig attracts thousands of people dancing two nights away by the sea.