Boise-area riders might be interested in a recently completed stretch of roadway - it made my day when I happened across it.
Gowen Road is a favorite loop, due to its proximity and general bike friendliness. (I say that despite the fact that our brother Jim Chu was recently killed on that road.) I occasionally will take a detour out Eisenmann Road (the road that fronts the Factory Outlet Mall), or Federal Way (Micron).
But always before I've had to do a U-turn and retrace my path, back to Gowen.
Well, my friends, as of quite recently, Eisenmann has been extended on out another half-mile or so, and finally hooks up with the I-84 interchange out there. (There's a huge new Winco warehouse a couple hundred yards west of Eisenmann, too.) So - you can ride out Gowen, turn south on Eisenmann and take it to the end, cross over I-84, and come back in on Federal Way.
From my house (near Kootenai/Roosevelt), it made for about a 20-mile loop. And conditions were perfect for a nice ride out that way today - overcast and cool.
[I post this information with confidence that my suggestion won't be life-changing for the cyclists who don't venture out of the north end, Hill Road, or the Greenbelt. Gowen and Eisenmann are no place for posers!]