Here's a few:
NW Aerials in Kirkland. $8 for the first hour and a half session, $20 for each after that. Best trampolines of any in the area, but they are not recessed into the ground and padding is pretty scarce. They are fun, but pretty scary to learn new tricks with. Worth going once for the other tramp floors and such.
Metropolitan Gymnastics in Kent. $8 for a two hour session, they have open gym 2-4pm Mon-Sat. They recently put new springs in the tramp, you can pull mats over to pad the area, and it's pretty deserted on weekdays. Best price for a trampoline that you can actually get some height on.
Sky High Sports in Bellevue. $10 per hour session. The best open hours of any jump place. The foam pit is fun, but the trampolines in general aren't very high powered, meaning that they don't give you much time to learn tricks. Still, it's a great workout and if you are looking to bounce off the trampoline walls, it's definitely worth going a few times. Watch out for the red pads though, there's some hard bars underneath them that hurt when you land wrong.
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