i'm thinking of using Rainier Ave & Renton-Maple Valley Rd (Hwy 169) to get there. (starting on cap hill)
any better way?
Well, for starters, I'd take the Cedar River Trail instead of 169, it goes as far as Maple Valley parallel to the highway but avoids the really narrow lanes, bad pavement and high traffic. You can hop on it in Renton just past where you exit the Airport access road, but it's hard to see since the trail is below the road and you have to look closely to tell that Airport way is crossing a river. First half mile is a little squirly, you have to cross back over the river on 3rd (just before going under I-405), but once you get on the official trail, it's straightforward.
After you cross Hwy 18, I don't think there's much logical choice except to ride on 169, but it's much more rural by then, so not too bad. If you don't mind a little extra distance (but you're asking because you're planning to ride to RAMROD, right? So you probably do...), a nice alternative is to turn left on Kent-Kangley road, then right on Retreat-Kannaskat road and thence to Veazle-Cumberland road, which eventually returns you to 169 a little ways north of Enumclaw.
Another alternative, slightly longer (i think) but possibly simpler and flatter(?), would be to ride south on 12th from capital hill, all the way to the end of beacon hill to where it drops you onto MLK, then drop down west on Ryan, cross I-5 and catch the Green River Trail/Interurban Trail all the way to Auburn. Turn left on Main street, then right on Auburn-Black Diamond road as you start to get out of town. Just after crossing hwy 18, take a right onto Green Valley road towards flaming geyser park. (Awesome country road riding here). Right on 218th, left on 368th, right on 236th, left on 400th, right on 244th stairsteps you down right into Enumclaw, all on low-traffic roads. (Check the map, it's not as complicated as it sounds)
Probably any of the cue sheets for the century rides to Flaming Geyser would be worth looking at, I've seen a few around on various websites.
I know the original post asked about getting to Enumclaw, but since you are doing RAMROD, but starting from Seattle you could ride South on the Interurban to Pacific and than hop on West Valley to Sumner and than follow the Sumner-Orting Hwy to Orting to connect with the RAMROD route just outside of Orting at the Kapowsin Hwy intersection. You're only missing a small section through Buckley and South Praire before joining the Hwy to Kapowsin. Just another option.
My Moto -- "I'm a cyclocross racer, I suffer and LOVE it."
I would also agree that the highway isn't bad between Maple Valley and Enumclaw for the most part