Picnics are the great. You can go to a park, go on a drive through the country and stop on the side of the road, or just anytime you're out. For the price of food you can enjoy a nice hour or so together.
Go to farms, there are u-pick farms in the summer for berries. In the fall there is all sorts of u-picks for pumpkins, fruits, and veggies. Pumpkin farms are great. You can find some with hay rides to the patch, corn mazes, a barn of hay, and animals to pet or feed. You can usually have a day of fun for about $20.
Camping is one of our favorites, where we can spend a whole weekend together. You'll have to invest in a tent for your family, they can range about $100 and up. Then weekends will usually be about $50 for Friday thru Sunday. Campgrounds range from parks that have very few amenities to resorts where there is swimming pools, miniature golf, arcade, ect. Most campgrounds have trails to hike, lakes to fish, sometimes if you lucky you can get some by the ocean. There is a whole world of camping gear you can buy for it, but you just really need a tent, maybe some sleeping bags but pillows and blankets will do at first. It's good to prepare meals in advance if you can, or have instant meals set up. If you have a good camping skillet things like hamburger helper are easy to make. We usually do hamburgers and hotdogs, doughnuts in the morning and sandwiches for lunch.
A day at the beach can be fun. Make sure you bring plenty of toys and sunscreen. Here is where another place a picnic would be nice. You can get a tan while the kids play in the sand. What out for small kids, sometimes the waves can knock them down and they can't get up. Some beaches have playgrounds nearby for a nice break.
Playgroups are a great way for moms to meet other people, and kids to make new friends. Just because you're a stay at home mom doesn't mean you actually have to stay at home. Try and get out at least once a week, its good for both you and the kids. There is a couple of groups that are designed for moms, look into what's in your local area.
Rec and community centers have great classes. Getting a kid involved in a sport is a good way to encourage your kid, met new friends, and enjoy a day together.
Crafts are fun for days you're stuck in. You can even bake cut out cookies, or other goodies. Painting a picture together, doing a puzzle, or just glue paper together. Whatever your kids get into, try it out, you may find that you like it too.
Family game nights are a nice way to bond. Weather its Candy land, Snakes and Ladders, or Scrabble, they can be a lot of fun. If you're good about picking games you can find some that are not only entertaining, but educational as well.