Is there a common route that Seattle GA pilots use to circle Mt Rainier while avoiding class B airspace? I'd like to fly around Mount Rainier in a C172 out of Renton (KRNT). What route do people usually take? I wonder if I can climb quickly enough once out of Seattle class B airspace for terrain around Mt Rainier. What do people usually do?
I fly out of BFI and RNT.  The short answer is go East to clear the B, then South to parallel terrain as you climb.  You will have more time to do this if there is a North flow.
There’s no specific route per se, just climb as best you can while keeping between the two.
The Lake Youngs departure gets you going in the right direction, and once just past Enumclaw you can make a circling climb to get comfortably above the ridges.