There are so many great playgrounds in Marin, one of my favorites is Corte Madera Town Park which has a great lay out with three sets of play structures for different age groups, from springy motorbikes for little tots on soft flooring to a climbing wall and a roller slide for the big kids. It has a sand box in the middle and a great circular path around the edge for riding bikes, all safely enclosed.
If you are looking for a park that the children can spend hours having little adventures while discovering new areas and hidden corners then check out the Millennium Playground in San Anselmo Memorial Park. It has a variety of complex designed castle fortresses that represent the Seminary Tower and Town Hall Tower which are a lot of fun to get lost in, with various slides that pop out of the structures. There is also a train depot with a wonderful wooden train, a giant dinosaur, sand box and swings.
For hot summer days, the Marinwood Park has lots of shade next to a BBQ area and the outdoor pool which has a tots pool surrounded by a grassy lounge area, great for play dates.
A hidden gem is the Palmisano Park in Hamilton, Novato, near the outdoor pool, which has great play structures and a large sand box with a tower in the middle with pulley systems for carrying the sand up and down. It is also our local park so a popular family hang out for us!