Last season she was a peculiarly bloodthirsty Little Red Riding Hood, and in a couple of months she’ll be warding off Nazis as Sally Bowles, so this may be just the moment to catch Megan Lawrence in a lighter mood. The Helen Hayes Award winner headlines a cabaret show called “Signature Sings Broadway”—the kickoff of that venue’s portion of Arlington’s summer-long free festival, “Arts al Fresco.” Tonight at 8 p.m. at Lubber Run Amphitheater, N. 2nd & N. Columbus Sts., Arlington. FREE. (703) 358-6966. (Bob Mondello)