How old are your kids? Mine are 3 and 8 months.
We go to the park if it's nice weather.
We also go to Manchester museum - free and fantastic. Not sure where you are, but I'm sure there will be some museums nearby. Some are better for kids than others.
We also like garden centres. Mind you, we do like the cafes there, so not necessarily free, or diet friendly! But if you can resist that, mine likes to run round the plants, there's lots for the baby to look at and it generally gets us out of the house.
Our Ikea has a brilliant play area. It's in the cafe, but you can get a coffee or a soft drink, and get free refills. Ours now has loads of the little kitchen things, a train track, a room with loads of cushions and CBeebies on a TV, and another area with other stuff in it. My daughter could seriously spend ALL day in there. Seriously.
Feeding the ducks.
Making stuff - painting, sticking, glueing. Evie will stick anything to anything. Give her a pritt stick and she's happy. Sometimes I cut stuff out of magazines for her to stick, other times she makes cards with sequins, stickers etc etc. We buy stuff from The Range, and it's not too expensive. I also buy packs of that glittery confetti from card shops.
If the weather is fine, I'm planning on doing some hand painting outside (yes, I'm feeling brave). I'm going to get a roll of wall lining paper, roll it out, put some paint on paper plates, put them in old clothes, give them some old sponges, brushes and whatever, and just let them go mad. Yep, I'm even going to get the baby doing it.