Lots of great suggestions here. Not sure if I've missed your kids ages or you just haven't posted them, but another thing I'd suggest is looking at what extra curricular activities are on offer in your local area, in school holidays and in term time, and see if there's any that could plug the gaps for you. Particularly if you're anxious about traveling anywhere, as you wouldn't want that to rub off on your kids!
For example, Scouting and Girlguiding usually offer pretty low cost weekends away if your kids are beavers/cubs/scouts/brownies/guides, or there may be a local holiday club which offers coach trips during the summer hols to the beach, forests etc.
Another suggestion may be to look at PGL activity holidays for your kids; generally these are residential, but are much cheaper than then entire family going for the equivalent time at Butlins and doing all the activities. You could even send your kids on separate weeks, so you can have 1:1 time at home with the other one.
I'd say (age depending) chat to your kids now about the big summer holidays. Ask them what they'd like to do, if there's anywhere they really wanna go, give them a budget and get them to plan a day out each. Then try and work it into a calendar. In your days just at home, having supplies for crafting, growing some plants, baking, and even cooking dinner sometimes, will help to ease the boredom.
You could even do a jar with colour coded sticks with activities on them - Eg yellow is at home, blue is local and free, red is further afield and more expensive, purple is local and costs some money, green is mystery.
(Yellow - bake crispy cakes, paint a masterpiece, build a fort and watch a film in it, pop popcorn on the hob, plant some seeds, tent day in the garden, board games, waterfight! Blue - visit the library, visit to local park 1, visit to local park 2, football on the green, cricket on the green, picnic in the woods, scavenger hunt, geocaching, Red - trampoline park, bowling, laser tag, arcade, mini golf. Purple - charity shop challenge, cinema, swimming, chippy tea, Green - invite a friend round, mystery dinner night, BBQ tea, stargazing at midnight, mystery outing, find a new park). Makes it easier to budget, and you can tell your kids to choose a free one, or to choose one for at home today.