For the younger, stroller bound kids, this is a great place to take them for a long stroll. The park connects to the Los Gatos Creek Trail which is paved and scenic, so you'll run into lots of joggers and bikers too.
Playground-wise, I've seen a lot better, but they did have a carousel which my 2 year old loves (at least for a couple minutes until he's dizzy). He also enjoyed the small bridge over the Lost Gatos Creek, but wouldn't have lasted long on the trail without a stroller.

I would have given this park 5 stars except for the fact that the two playgrounds are separated by almost a quarter mile path/field. I went with two kids around 2 years old and they loved both playgrounds, just not the walk in-between them.

The first side we went to had a massive play structure that stood over 25 feet tall - this is more suitable for slightly older kids than ours, but still lots of fun nonetheless. For the toddler set, the play area for them was significantly smaller and they can't help but look up in awe at the multilevel older kid park right next to their sand box.

If you're looking for a place to cool off, the playground on the other side has colorful and fun water features for kids of all ages. Both sides have picnic areas and the park has lots of fields in between for all sorts of games and sports - or a long enough walk to really tire out your toddler. Next time I'll bring a stroller.

We brought our 2 year old son here to walk/run around a bit before lunch and he had a lot of fun meandering along the well maintained walkway/trails that led down to the water. Our son wasn't interested in any of the sculptures at all, but just enjoyed running down the paths, looking at people and other kids, and the triple whammy of watching boats, airplanes, and trains all from one place! On a nice day, this is a highly recommended place to go out for a pleasant and relaxing stroll.

On our trip up to Seattle with our 2 year old, we were treated by our friends to a boat ride around Lake Union. The Electric Boat Company rents out electric boats that can seat 10 adults comfortably (min. 2 hour rental) and allows you to captain your own boat as you check out sights around Seattle from the water.
Bring along some food (or have it catered) as well as CD's for music, and this is a relaxing and memorable way to see some sights with family and friends. There were 3 kids in the boat with us, a 1 1/2 year old, our 2 year old, and a 3 year old - all had a good time, though the younger 2 were lulled to sleep halfway through the ride. What makes this area particularly exciting for the kids are the sea planes that take off and land right on the water next to you, as well as all the other boats (you can even go down to the Ballard Locks).
The only catch is that a 2 hour ride on a small boat might be a tad long if your children are very young unless they can easily fall asleep.

I can't say enough about Wallingford Playfield! I was introduced to the park during a visit with my 2 year old, and he could have stayed there the entire day or even weekend, if I'd let him.
It has a number of different play structures as many parks do, but these seemed very well maintained (almost brand new) and had a good variety of things to occupy the kids. Aside from the play structures featuring several different slides, it had swings, a carousel, sand pit and digger, etc. What set this place apart though was a good sized wading pool for the toddler set, complete with stones for jumping across and a hidden little hiking trail for the more adventurous. Picnic tables, nice rest room facilities, multiple fields, etc. - pretty much anything you could ask for in a neighborhood park was here. This will be the first place our son will want to go to on our next trip up here.