A few creative and different ones: Martin's Park near the Children's Museum. Alexander Kemp playground in Cambridge. North Point Park in Cambridge near the MoS.

There are a lot of good ones in Newton and Brookline, like Newton Center Playground (basically 3 playgrounds in 1), Albemarle Playground (great climbing structure), Soule Recreation Center (really good toddler area, another area for bigger kids and a splashpad), Pierce Playground, the new one at Ridley school (fka Devotion), etc.

We like the zip lines at Bobby Conners in Waltham and the Esplanade Playground (also a cool climbing structure for bigger kids). The slides at Skyline Park in Arlington. Artesani in Allston and Beaver Brook in Belmont during the summer for their water features.

Saw someone else recommend a farm, so I'll mention Connors Farm in Danvers even though it's not free.