Depends on the neighborhood. Powell Barnett in the Central District is probably the best in the city!

You Guys!?!?
How can you forget Madison Park? It was redeveloped a few years ago now by someone with a mean sense of fun. Lots of literally twisted play structures and many spinning devices and a Zip-line. Plus it has the highest slide in Seattle that I know of.

Walking distance to a beach, tennis courts, Red Apple, Ice Cream (Scoop DuJour - I can't spell that) and Mad Pizza delivers.

I second Cal Anderson Park on Capitol Hill. Cool water features, including the "Fountain Mountain", rubbery bouncy playground floor, and right across Pine Street to Molly Moon's, or better yet Bluebird Ice Cream on Pike Street where besides ice cream they have loads of games like Operation and Mouse Trap, and you can always plan around an art project with some eyeball stickers at Blick's on Pine & Broadway.

Take them to Carkeek Park. Beach, railroad tracks (not for playing on, for watching), creek, miles of hiking trails. Oh yeah, and there's a playground, too.

I like the playground in Madrona. It is small, but you can take the kids for a cupcake and hot chocolate right across the street. They have rings and swings, best for little kids though. I think the 10 year old boy would be bored and the five year old would love it.

The Martin Luther King playground on MLK and Jefferson is unique in that it has a giant climbing structure made of rope. It has a ton of stuff to please both your age groups.

Cal Anderson is a goodie because it has a decent playground plus the water features in the park.

Greenlake too. Great playground with huge sand pit, then when you get bored of the playground you can amble over to the lake.

We are blessed with so many good parks in and around the area.

I recommend (in no particular order):

1. Seward Park. They put in a new park last year. It is great for a range of kids and includes a zip line. There is also a swim area, paved bike path and a nature center complete with an African tortoise.

2. Jefferson Park on Beacon. This is also a brand new park. Great equipment, adjacent basketball and tennis ball courts, paved bike path and a great field for kites.

3. Saint Edward State Park on the Eastside. It's a bit of a distance, but boy is it worth it. Huge play area, waterfront beach, activities galore.

3 year olds don't "need" classes. Life is a class. Drag him around outside instead - he's going to spend most of his life inside, so you may as well save some money and get him out in the dirt.

Eating Dirt 101 (Improves their immune system as long as it isn't filthy dirt).

That and various assorted music and language classes. Maybe introduce them to foreign languages. I spoke and read German before I spoke and read English.

I don't have personal experience with them, but I hear great things from my students and their parents about The Moonpaper Tent
( http://www.moonpapertent.org/index.html ). They have a lot going on in the art/dance/theater arena, and are located in the Ravenna area near Whole Foods. I walk by on my way to the doctor's office pretty regularly and the kids always look like they're having a blast.

There are some great music classes out there. I just took one with my 19-month-old at the Phinney Neighborhood Center. There are also great music and dance classes at some of the Community Centers. And I know Music Center Northwest has great offerings.

When I was in high school, me and my brother and sister and two dogs would all cram onto my parents bed in the morning and make them watch Napoleon Dynamite with us.

Similarly, when I visit home I love having movie night with my younger siblings...I make a huge spread of appetizer-like foods and make it all fancy like it's a party. They love it!

The zoo is always fun. Cheap in the winter. Just dress warm.

Movies are a good form of entertainment if you need to keep occupied for a night. Not really sure what's out now though.

The Pacific Science Center is a good place to spend the afternoon. I think they still have the Harry Potter exhibit up. If it sounds fun to the 15 year old you should be good.

Family Fun Center?

Mountlake Terrace is an awesome facility, but I give all my votes to Safe n Sound Swimming on Lake Union. http://www.snsswim.com/

Have you contacted the YMCA?

(I'm serious. They are the experts when it comes to swim lessons.)

Mountlake Terrace has a wonderful facility and I've heard great things about their lessons (ages 9 months and older).