Islay Hills Park in San Luis Obispo is an enjoyable park to spend the afternoon at. There are 2 main playing areas for the kids- one for younger kids and one for older. A few times that we've been there, the older kids were playing rough on the smaller play area, so watch out for that. We like the fact that this park isn't as crowded as some of the other SLO parks are.
Islay Park has a lot for kids to do: two great play structures, swings, sand for digging, paved trails for walking or biking and a baseball diamond and fields for running around. Plus, it's a very open park so it's easy to keep an eye on your children. The park can get hot in the summer (and there's not much shade), but it is very pleasant most of the year.
Islay Hills Park is out of the way, and maybe that is why it is usually not too crowded. The park has two play structures, one aimed at older kids, the other for smaller kids. There are also tennis courts and basketball courts, as well as a baseball field. There is a walking/jogging trail that takes you along the base of Islay Hill as well.
I used to take the kids I nannied to this park all the time. It has two play structures, one for older kids and one for younger. Their is also a level bike path that you can see from the playgrounds so if someone wants to ride and someone wants to play, you can feel comfortable letting them do either. Their are also tennis courts and decent restrooms, a soda machine and basketball courts. It's a lot of fun!