I did this in my local playground.
It was a few years ago. We got grants to fund it, but I think they were all EU, so you're fucked for that now
The grants, although amazing, are a pain and massively time-consuming.
I would recommend planning for equip older than your DCs current age if you want them to get the benefit.
Some kind of climbing frame is v popular. Esp one they can balance on or walk along.
We got a load of posts like high up stepping stones. Those were good.
Zip wires are over-rated, because there's only one go at a time, and if there's a CF around who gives their kid multiple goes whilst a queue forms, people will get cross.
Our best thing was a 'nest' swing.... a single swing, but it's a big circle with netting. They can lay in it while you push. And they can all pile on. And teens and adults can use it too.
Our kids REALLY wanted a climbing wall, which we gave them. But after they scaled it (took approx 30 seconds) they never bothered again!
Hope this is helpful