We booked a last minute concert series spot when a dueling pianos band backed out/fell through. We set up in the bandshell in a park. The gig was out-of-town for us (in the town I grew up in), and the forecast was a little iffy--30-40% chance of rain off and on all night. As we're setting up, it's raining. People are showing up in parkas and with umbrellas (this is a every-week in the summer thing, so folks were showing up for the concert series and not us). It's drizzling. As we start to play, it starts pouring.
There were maybe 50 people there (booker said they get 500-600 usually). We lost almost half of them when the skies opened, but the rest of them were bound and determined to see their free concert. What do we do? We pulled ouselves up to the back of the bandshell, turned the mains way down, and invited everybody to come under the bandshell with us. So we played to maybe 30 people, all in lawn chairs within about 10 feet of us, all up on stage. It worked, they want us back, we sold a handfull of CDs, etc... Good times overall.