Autistic preschooler here. We have a small low slide. Not tall enough to hurt falling off but still satisfying.
And lots of steps. Steps up steps down, upside down plant pots to step on, big flat stones, bits of wooden decking. Steps up steps down. Plastic stepping stone trails are really popular at nursery (and great for vestibular input/balance practice) so we are thinking of getting some.
A pot with rustly grasses and bamboo to stick fingers in and shake (and snap and break but it grows back).
A little mini fountain. No standing water but it just trickles water out of the top and it all catches at the bottom but you can stick your fingers or faces into the trickling bit.
Lots and lots of edible plants. Gradually replacing all the plants with edible ones.
Other thing popular at nursery is lengths of guttering attached to the wall in a zig zag pattern. You can roll balls down it or pour water down it (or just lick it) and apparently it’s hand flappingly exciting! I’m planning to make one by fixing a piece of trellis to the wall sideways then fixing stuff to that.
Wind spinners. Twisty things hanging in the tree, windmill things planted in the ground. Great for flapping and spinning.
If you’re worried about two children at once on the trampoline then maybe look at an in-ground one? Harder to hurt yourself falling off although not impossible.
Lavender and rosemary and other calming herbs.
And some shelter - we have a bus shelter style seat thing; you can sit on it when it’s raining and not get drenched. Handy for me not to drown when child is insisting on being out but also very satisfying to sit in and feel the rain with fingertips or toes and not the rest of you.
We have a sort of trail through the garden which involves stepping, jumping, spinning... I didn’t design it that way but my child has created it through using it all the time. It weaves through the flowerbed too but apparently going round it over and over again is very satisfying. It includes the slide and then pushing the swing, climbing up onto the bench abs jumping off, all the way round the tree, and so on. Repeat. Not a huge garden but lots of exercising can be done following the trail in the right order.