My son is 3 and he only goes to pre school for 2 mornings so I have a lot of time to fill with him!
Depends on the child but mine is active and needs a lot of exercise.
I don't like softplay or parks overmuch so we tend to do lots of everyday things and I find he gets more from that than from the 'organised fun' for kids. (His dad does a park or 2 with him thank goodness!) I also find our library beyond dull but we both love reading so we go to waterstones instead and look at their fish in a tank.
He loves a supermarket trip and pushes the trolley for me, helps me choose things , scans the items and even gets my card from my pocket to pay!
We walk around our local town and we do lots of pushing traffic light buttons, going into shops, escalators lifts etc.
National trust places, we've got a few transport museums near us with planes and helicopters taking off and landing ( and lots of room to run around without bothering anyone), garden centres, walks in the countryside, swimming lessons.
At home, every toy and game possible, throwing and catching balls up and down stairs, tonies box, pretending to drive my car, riding a bike, bit of gardening, 'helping' with housework. Paddling pool, lawnmower, sand, garden tent etc in summer.
Mostly, he loves it when I'm busy and he can bustle around with me and chat to me and his toys. I cleaned a room yesterday while he sat in the washing basket and pretended to be blasting into space - I gave it a good rock for him in-between dusting and I think it was his favourite half hour this week!!