Hi all,
Good rides both and plenty more great rides up in this area too.
I've done the STP for the past 5 years; first time ('99), I did it with a few friends in 2 days. It was very social and a lot of fun, but I liked the idea of challenging myself in riding a double century in 1 day, so have done that for the past 4. Usually about 8,000 riders and maybe 1,500 are 1-day riders. No matter which option you choose, it isn't the kind of route that needs hill training; instead you train for mileage and learn how to pace yourself, managing your rest stops, food, hydrating needs effectively.
I worked a few aspects of riding it in 1 day to my advantage, for example: the crowds thin out about halfway, when you spot 2 day riders grabbing their gear and heading to their overnight spot, so it opened up the road; no lines at all at porta-potties the rest of the way! It's great to get in, get a shower, massage, dinner and great sleep on Saturday, then stroll over to the finish line festival's beer garden on Sunday, after a lazy brunch - sporting either your 1-day rider patch or t-shirt - or both!
If you pace yourself and are efficient about how you train, what pace you want to maintain, don't stop at every opportunity but instead maybe every 50 miles, keep drinking, eating, stretching even while on your bike, it's a breeze!
The Chilly Hilly is the long-standing (25ish years) official opening ride to the season put on by the Cascade Bicycle Club. There are bike only ferries for the hordes of riders heading to Bainbridge Island from the Seattle side and it's a very festive atmosphere and well-supported ride. Beware though - the name is appropriate! It's usually pretty chilly, at least at the start and since the route generally follows the perimeter of the island (33), it's a hilly one. Lots of short ones, and rollers and some long pulls but none of it stops a few thousand riders of all types and on all kinds of bikes from coming out to have fun.
Someone also mentioned Vashon Island on this thread and that's a very popular loop for area cyclists too - with a longer, steeper ferry hill than Bainbridge even.
Have fun!