The Woods Hole Aquarium is free. It is small but there is a seal tank with a few seals in a tank in front of the aquarium. They have a touch tank and lots of aquarium tanks with fish. Woods Hole is also a nice place to walk around. Parking is a nightmare in the summer. There are meters on the street so you'll have to have quarters. I would much rather go here than Zooquarium.
Coonamessett farm is a cool place to go. They have some animals and you can pick your own veggies and berries (if in season) They have a small store, and an ice cream stand. You can get breakfast or lunch and sit on their outside deck and eat. Kids love it here. It's $8.00 per person for a day pass. If you would like to go I have 4 free passes I can send you.
www.coonamessettfarm.com
In Sandwich there is a fish hatchery that is free. They have the gumball type machines that have fish food in them. It's a quarter. Kids like to feed the fish.
The Sandwich Glass Museum in Sandwich is $5.00 for adults, $1.25 for kids 6-14. They have glass blowing demonstrations and a movie on the history of glassmaking in Sandwich. At the front des they have a paper that they give kids for a scavenger hunt. They have to look through the glass displays and find certain items. If they bring it back and they find all of the items they get a few marbles.
http://www.sandwichglassmuseum.org/index.php
The Sandwich Boardwalk is fun. You can walk the boardwalk to a (rocky) ocean beach. There are little coves where the kids can play in shallow water before you get to the beach. It is beautiful. I think it is $10.00 a day to park. Lots of teenagers jump off the boardwalk during high tide. There is a horrible rip current. Grownups fight their way back to the ladder sometimes.
If you are going to Hyannis there is a chip factory that is free. (Cape Cod Potato Chip factory) It is nothing spectacular. The whole process from potatoes in boxes to bagged chips can be seen from a hallway about 50 feet. You are empted out into a small gift shop. Each person does get to take 2 small bags of chips for free.
The Knob in Falmouth is a nice walk. It's on the way to Woods Hole. You walk through woodsy paths and end up at a giant rock that you climb. (stairs) It's right on the water and the view is spectacular! The walk there is short.
Get a copy of Friday's Cape Cod Times. They have a pull out section of everything going on during the week. There is usually a lot going on in August.