Our half term was last week and we were all ill, so did very , very ittle other than had a lot of screen time!
However, don't feel the pressure to spend money to do things.
Have you got a local park/nice walk place? In our half terms we usually do at least one day at a national trust place (we pay £11 a month for a family membership for myself and husband, 7 year old and toddler). We take food, drinks with us - so a super cheap day. Sometimes we do two of them in a week!
We usually also do a day where we go to a large local nature trail/walk which has a play park. We take food there too, sometimes a football and if we can meet friends there then even better.
A swim one day for under £10 is usually a great activity. A day at home baking/playing games/doing Lego/a movie afternoon or even a day at Grandparents to do similar always goes down well here too.
My general rule in holidays is that screen time is much more relaxed as long as we are also active. Eg - if we've walked the dog and been for a swim in a morning then I'm completely fine with a netflix binge in the afternoon, then Lego/books between dinner and bedtime.
Meeting up with friends for a playdate or in a park is always fun and free to do!
Don't put yourself under pressure or assume that you have to spend money to have fun.