Your children have already had a big trip - camping. A day out with friends is another great day.
In terms of the rest of the holidays. If we continue getting great weather it is so much easier to do very low cost/ free things. Especially if you can get a few friends in with it. Maximise parks, free museums and librarys, car park only pricing venues.
If you really want to do a day out look at country parks/ seaside or places where you only pay for car parking. Pack up food and drinks. I've found since COL has bitten and everyone is packing up the cost of drinks and ice cream at many venues has shot up. As a PP suggested, pop into a shop and grab a box of ice creams to share.
The key to keeping costs down but still getting some "big ticket" days out is planning. We do quite a few "big" days out, I have never paid full price for any of them. If we do a big ticket day then we are packing up and keeping the rest as cheap as possible - no gift shop for example.
Scour FB/ social media/ Groupon/ days out with kids ect for local venues and see if any of them are offering discounts, giveaways. If you have anything like NT/ EH/ Annual passes make sure you are fully using those for free days out. Also check term dates of neighbouring counties, for example our Easter holidays are a week earlier then our neighbouring counties, therefore last week we were able to go to an attraction for term time prices instead of Easter holiday prices. The same often happens in May and you often get a few days either side of the Summer holidays that are different - see if any of these days are still "term time pricing".
I agree with a PP suggestion of prepping for May and Summer holidays. Put x amount a week away if you can. Start looking at places you may like to visit and see if anywhere is offering early bird prices or is doing Groupon offers or very cheap open days. Having a few days already paid down in the Summer holidays is a huge help. If you are on social media following what's on in the local area pages for kids can really help you plan. Depending on kids ages and where you are, a NT pass can really help over the summer holidays - you can do a monthly DD and cancel it when no longer needed - they almost always have something on at most properties in the summer for kids.