Yep get one, we did and the kids love it. Get the largest you can afford or fit (unobtrusively) in the garden. I think the most we’ve had in ours (10 ft) so far is 9, from 2 years to 17.
Edit: And as above, it’s fresh air and exercise, what’s not to like! (Neighbours might disagree).
Edit 2: As for the rest of the garden I just cleared a corner of ours that had large rocks randomly dispersed (not quite a rockery) and piled them all up. 5 minutes after I finished piling them up it was covered in cars and action figures (2 boys, aged 3 and 4) – plenty of ‘natural’ things for them to do in a garden.