I'd invest in the cover, if you can get one in time. Our Babies R Us had them, but I ordered one from Walmart.com that fit my stroller because it was a bit cheaper. The baby stayed drier than we did! The cover fits very close to the stroller, so no worries about the wind blowing it up, and it has some nice vents so I didn't need to worry about the baby getting suffocated or too hot. Also, it comes down far enough that we were able to put our bags in the stroller basket and they stayed dry, as well. We would stick our stuff on the seat of the stroller when we went indoors or on a ride, and didn't have to worry about a sudden shower ruining everything before we got back to it. If you don't have time to get the cover, then take some clothespins with you and use them to hold the poncho down so the wind can't whip it up. Or maybe see if you can get a plastic tablecloth - that might cover a double stroller better than a poncho would.