Mostly, capped landfills remind me of the mausoleums of a consumer society. For most of a century we dumped our solid waste onto these Mt Trashmores and mixed up a brew of concentrated toxins which seeped into the surrounding areas and often polluted (and still do) our water. So we learned to treat our waste as a resource, close the landfills, cap them, and leave them idle. We’re still very primitive about this, but progress is steady.
There’s not much you can do on a capped landfill because it’s essential that we not disturb the protective rubber liner that is usually only 12-18” below the grass that covers it.
But there are some uses. Most are relatively passive: cultivation of hay, green space, wildlife habitat, and biking/walking/running trails. Some are more active: golf courses, baseball fields, and soccer fields.