See what the Foundry could be in a giant, one-day art show and open house. The arts community is trying to jump-start the city process of figuring out what this 52,000-square-foot former manufacturing space can be. At least 10,000 square feet must be community space, and the community is eager to have it – or more. But little has happened with the Foundry in the year and a half of city ownership, and organizers hope this one-time, four-hour Foundry Equation open house will energize the argument by showing the possibilities. The space has been filling with contemporary and children’s art installations over the week, and there will be live music and improv performances. It’s free and open to residents, arts enthusiasts and the community at large from 11 a.m. to 3 p.m. Saturday. Its website is here; organizers are encouraging use of the hashtag  #thefoundryequation.

Celebrate the summer solstice musically with the Fête de la Musique. Inspired by a street-music festival started 29 years ago in Paris, the sixth annual Make Music Harvard Square/Fête de la Musique brings more than 110 bands and musicians from across the country and Canada to play on 14 different stages throughout Harvard Square – everything from Rock ’n’ Roll, funk and folk to jazz, country, hip-hop, some standards and classical, pop, R&B, soul, alternative, goth, bluegrass, world and, um, grunge, if that’s still a thing. This sprawling festival of music and dance takes place from 2 to 10 p.m. Saturday in the square’s parks, plazas and sidewalks. Find a full schedule here or just follow the music.

Make some music of your own at an old-fashioned sing-along. Join your voice and musical talents with others in an all-ages sing-along. People are invited to share songs or take inspiration from the songbook “Rise Up Singing” as well as to bring guitar, percussion or any other instruments to add to the celebration – and to spread out a picnic and enjoy. The event takes place from 6 to 7:30 p.m. Saturday at the Somerville Community Growing Center, 22 Vinal Ave., near Union Square, Somerville. For information, e-mail Liza at [email protected].

Let the Cambridge Symphony Orchestra pops concert take you from “Star Trek” to John Philip Sousa. This free Pops on the Lawn concert has the Cambridge Symphony Orchestra, led by music director Cynthia Woods, performing crowd pleasers of every sort: Sci-fi movie themes from “Star Trek” and “Superman” will be played alongside patriotic music, Broadway shows such as “West Side Story” and classical favorites by Brahms and Copland. Children will be encouraged to conduct along with marches by John Philip Sousa – as though they could be stopped. This concert, supported by a Volunteer Ventures grant from the MetLife Foundation, is at 3 p.m. at Sennott Park (Broadway at Norfolk Street, near Central Square). Attendees should bring a blanket or lawn chair – unless the weather gets bad, in which case the event moves to the Area IV Youth Center, 243 Harvard St., next to the park.

Experience the Revenge of the Robot Battle Nuns! The organizers of this multi-arts burlesque circus probably say it best: “End of the World. Foxy hallucinations. The president’s mutant daughter. Mad science. Cannibals. And one bloody, perverse story of revenge. Who amongst you has a taste for the bizarre? An eager ear for this terrible tale? For those with a strong stomach, a twisted mind and a perverse groin, REVENGE of the ROBOT BATTLE NUNS has it all. Aerialists, stilt walking, gender bending, boylesque, splatter, a rocking soundtrack, comedy, violence, and, of course, pasties. So what does happen when two people who hate each other the most end up stuck together in the end of the world? What is the sexy Scout’s dark secret? What do you see if you lick a nuclear frog woman? What kind of welcome do you receive from a Lesbian Spider Cult? And WHAT THE FRICK are Robot Battle Nuns?!? Curiousity piqued? Nerves shaking? Pants tightening? Well, don’t worry, apocalyptic cuties … The Robot Battle Nuns are closer than you think. And they’re coming … For you.”

Presented by The Slaughterhouse Sweethearts, the show features burlesque talent from behind every red velvet curtain in the area, including the Sweethearts’ own Femme Bones, Belle Gunz, Anne Frankenstein and Sake Toomey; UnAmerika’s Sweetheart Karin Webb; The Bitches of Destiny; Abby Normal and Honey Pie, from The Babes in Boinkland; and more. It runs at 8 p.m. Sunday at Oberon, 2 Arrow St., Harvard Square and repeats June 30. Tickets are $20 for table seats or $15 for stool or standing room.