I mountain bike.
If I could do it everyday (I can't, live in the northeast) I would never play FFXI again. I literally wouldn't do anything except be out on my bike if I could, but that's not feasible.
What kind of barrier for entry is there for mountain biking? I ride my road bike whenever the weather allows (which isn't that often living in Seattle) but would like to venture into mountain biking as well.
The PNW is definitely a great place to mountain bike. In fact, Bellingham is only about an hour to 1.5hr north of Seattle, and it has some of the best mountain biking in the country. Of course, the whole PNW region is known for it's mountain biking, from northern CA all the way up to Costal BC and Squamish.
Barrier for entry isn't much, and your fitness from road biking will definitely help you. Decent entry level bikes can be had today for 500-700+. Decent entry level full suspension bikes can run you under 1500-2500+ and then up from there. It's more about your budget and what type of bike / terrain you're riding. Start with some easy trails and work your way up.
Definitely check out some local bike shops and ask around. But you could get yourself a Norco or Kona hardtail living in Seattle for like $600-700 and end up with a decent amount of bike for the money. Brands like Intense, YT, Canyon, Diamondback, and Transition (a Bellingham brand) have all gone direct to consumer now, so if you wanted to spend more money and get as much bike as possible – that's the direction I would recommend.
If you're worried about buying a bike and not being into mountain biking at all, buy from REI. I don't recommend you use REI as your bike shop, in fact I would take any bike I buy there to a proper shop to be adjusted, but they have a 1 year return policy and you can actually ride the bike before returning it.
All you need is a bike, helmet, hydration, and proper shoes and you're set to get started.