Inhouse Field Trips

I've also gone the post office, grocery stores, and pizza parlors (although when we went to Fellini's they charged).

Check your local metro parks - they often offer free or very close to free activities for many levels and interests for kids and adults.

You might also try geocaching or letterboxing. It's like a treasure hunt. Geocaching gives you gps coordinates and you have to try to find the cache (http://www.geocaching.com); you can download a simple app, or use a compass or GPS to do it. The letterboxing is done with clues and you use a stamp to mark when you find the cache (http://www.letterboxing.org/). They are basically free activities.

Plan some simple things, like an obstacle course at home or a nearby park. Take a ball, bucket, some sticks, frisbee, side walk chalk, etc. and make an obstacle course.

Plan some simple science experiments, like make a volcano with vinegar and baking soda, etc.

Host a party for your kids and friends. Make a home-made pinata, and include games and small prizes, or do a scavenger hunt.

Check to see if your local zoo has discounted days (ours offers 1 free day per month, usually a Wed, if you live in the county the zoo is located in).

Check out the library and your local rec centers for activities.

Consider doing a week of "camp" at home. Pick some camp activities and do home versions.

I read the comment about creating a trip in your room, and second it. One year we used a space theme and created a walk in space module from free boxes. Another year the room became the bottom of the ocean.

As for free places to go, our kids always loved the walking downtown field trip. And the place the children liked the best was the local hardware store! Honestly, they could spend hours examining all the types of chain and piping! And the hardware store employees and owner were so very gracious answering questions. This required lots of supervision, but was well worth it. Everyone learned tons, and now I'm very comfortable browsing for my household needs in that same hardware store!

Edit: I just reread your question and noticed you asked for in-house only. Sorry! I would definitely ask parents for connections. All my best.

Call the mobile snow cone people (Kona Ice around here) and see if they will come do snow cones. The kids can bring cash that day (have a set size they will be getting). They kids can all sit outside and eat them. I don't know what ages you have but what about water activities ? Sprinklers, water tables, etc. Have the kids put on a play and work on it. Then at the end of the summer have a presentation and maybe a party.

Your local high school or parks and rec might have groups or teams that would be willing to volunteer their time to do a performance or a short coaching session (cheer leaders, soccer, dance group, band, etc). This is the kind of thing the drama department at my University would love to do! Many times students need to have volunteer hours and would be willing!

Also, you might be able to talk some people into donating their performance instead of charging you (magician etc.)

Think about people you know who might have a free day to come visit. A friend who works at a restaurant can teach the kids how to "cook" a simple dish. Someone crafty can teach an art class. Or combine the two and do a cupcake decorating class! A grocer or farmer can teach about produce and maybe bring a sampling of some healthy fruits or veggies.

You can also transform the room for a day, but not actually bring in someone. For example, you can turn the classroom into an Art museum by hanging art all over in a gallery style. Don't tell the kids about it, and surprise them when they walk in. Act like you have completely left to classroom and went to an actual museum.

Libraries are great during the summer. The field trip usually includes a summer reading program intro which gets the kids excited to read over the summer.
Also local hospitals will sometimes do tours of the children's wards.
Local upper level schools are unusually willing to give tours to younger kids to show them what life in high school is like.
Local politicians love kids! Try your civic offices or local party offices
Courthouses also make for interesting tours if you catch them on a slower period.

Hope this helps

Ask your high school if they have a summer program that would come perform- drama or band. See if there is a local gymnastics team who would do a demonstration and small clinic. Ask a local scientist to come do experiments with the kids. An architect could talk about familiar structures then do a little building activity with blocks. Many hospitals offer outreach programs that teach kids about safety or health, so check there too. And don't forget to ask the parents- they may have a talent or connection that you haven't thought of.

The library usually have a summer program with free events

Many bowling alley's are doing free bowling including shoe rental and lots of movie theaters are doing free movies during the summer. What about a trip to the beach or park with lots of water involved games like water balloon toss, supper soakers that sort of thing?

County forest preserve and local musuem
Local Businesses - landscaping and manufacturing companies
College students - drama students and science students

Good Luck!