Hellobee Boards

The closest one is wood chips. That's my favorite because it seems like the least dirty and it seems to absorb rain pretty quickly so it's not just a mud puddle. The second closest has a rubberized (?) surface. But there's a sandbox in the vicinity so the whole thing is coated in sand, which is extremely dirty and becomes slippery. I've never seen pea gravel here but I remember it from childhood!

Sand.
So messy.

Small pebbles/rocks at the one walkable from our house.

The other one we usually drive to has the modern absorbent/soft-ish mats.

We have one park we really like that's rubber padding with mulch just under the swing. I wish they'd just put rubber all over though, as the mulch kind of migrates. Another park we end up at more often is just all mulch.

His playground at daycare is sand and I HATE it. He comes home with loads of sand in his shoes every day. If we don't get to his shoes first, he'll take them off and make a huge mess with them. I actually just bought a certain type of sneaker for him that are made out of crocs-like material so that we can just shake them out and rinse them or wash them if we need to, because the sand is so horrendous and would never get out of his regular sneakers.

The playground down the block has mulch. It is not so bad now, but before she started walking well, I really hated it. Because it isn't an even surface, she would stumble on it and fall constantly. I had to pick wood splinters out of her skin, clothes, and diapers.

Our favorite playground has a flat springy rubber surface.

Sand, grass, concrete, and that padded rubber asphalt material.

all the city playgrounds in our area have rubber ground cover to make them wheelchair accessible. There are also areas with sand which are the designated sandboxes.

All of our walkable playgrounds have those soft-ish, rubberized mats that look like asphalt but are squishy. There are some asphalt areas (in the water play area at the one closest to us, which makes no sense to me, but whatever!)

There's one by DH's office that is sand. I'm actually glad we decided not to live in that neighborhood because it would have driven me bonkers.

Mulch. I hate it.

@smocks: Ticks can only move across surfaces that are really dry. So if you have a playground that adjoins some woods in an area with ticks, a wood covered ground would not be ideal. If your playground is out in the open, not near an area with ticks, I think it's unlikely that it would harbor any regardless of the ground cover.