Pull a couple off the Try! album. The Ray Charles "I gotta woman" cover, done in a blues trio style is cool (but I guess that doesn't work because Myra would be singing), and "Out of my Mind" is a fertile playground for improv. "Come when I call" and "Everyday I have the Blues" off the recent live album also have lots of room for improv and expression and I bet your 4 could do a great job of it. Myra could really get somewhere vocally on those I bet.