Example: Shoreline Amphitheater, near Google HQ in Mountain View. Built on top of a landfill. For a while in the 80s, there were occasionally small fires during shows when people lit cigarettes. Google also harvested the methane and used it to power some stuff, although I can't find an authoritative article with details.

Indeed, sometimes big things. The landfill we used when I was growing up is now beneath a Home Depot, which was built over the top of it almost 25 years ago. The landfill in this case was unlined, too.

Yup. It is a little undesirable for various reasons, and not every landfill is suitable for construction on top (seismics, sealing/capping technique, materials, etc).

Like ski courses!

Several schools I know of have part of their playing fields on reclaimed land that's former landfill. You can't build on it because you have no idea what sort of gases and possibly toxins will work their way up from below, but wide open fields with free air movement that aren't round-the-clock occupied are fine. The only downside is that for an initial period you need to re-cap places with soil every few years as the fill underneath settles. There was one place where they'd paved over it rather than leaving it as soil to create tennis courts and after a few years it was a sort of dune landscape since they couldn't backfill the dips with soil. It was quite picturesque actually, a sort of post-apocalyptic look with miniature ponds with reeds growing in them and occasional visits from ducks. Sure, they'd lose a kid in one from time to time, but being a large school they didn't have a shortage of those.