We do little daytrips all summer long.
Lighthouses are big here (we live in Michigan), but there are state parks and other nice things that are free or very low cost.
We have a state park 45 minutes from here that has 282 stairs to climb. It has a great view, it's free, and if you run down the sand dune on the other side, you are at a Lake Michigan beach. We pack a picnic lunch and go for the day. There are also 3 letterboxes hidden at the top, so we plan to work those into our visit as well. We actually hope to visit this place a couple times this summer.
With the lighthouses, we take pictures at each one (some offer tours, some do not). We have a scrapbook that details each visit. Some are daytrips, but we have also done overnights to visit several in one trip (it depends on how much time off DH can take). We stay at low-cost hotels/motels and bring almost all of our own food. (We haven't done an overnight since youngest DD was born, she doesn't like the car at all).
The beach is always a daytrip, we only live 25 miles or so from Lake Michigan. We always pack a picnic lunch and hang out for several hours playing in the water and sand. We visit beaches that aren't very busy and most have a park nearby, so we play there too.
Fishing is a daytrip, we have many lakes within 20 miles of our house. We pick a lake, pack lunches and take the boat out for a few hours. It's a great free trip, fun family time, and great scenery...plus we usually catch dinner
Kayaking is also a daytrip...sometimes longer than we plan. We have river maps and we plan our trips with that. We put in at a bridge near town and then plan on kayaking to the bridge in another town, where we leave a vehicle before hand. We pack lunches and snacks (and water), and take off for the day. We have planned a 4 hour trip that took 10 hours before...that river map wasn't very accurate. LOL This will be the first year that DD6 will go along with us on these trips. DH and I would usually go while she was visiting her grandparents.
That's all I can think of right now, but I am sure there are more. We are very frugal, but like to get out and do things too. Gas prices are slowing us down a bit, but we hope to still do a few things this summer.