Your favourite family activities/days out that cost very little

[Mumof1DS]

I am looking for some inspiration. What are your favourite family days out? Or activities? Are there any favourite activities you enjoy inside?
The caveat is that it can't cost very much/anything!

I like going to national trust sites and walking. Around the grounds,a local one to me has a lake and old ruins from some sort of castle type building. Also there's a free adventure playground at a local airfield site. My mum takes my DC to the library as they always have things to do also I take him swimming as I'm a member of the gym and it's free for him. He's 3.