I took the girls to South Bass Island (in Lake Erie) last week and I took my Canon 5D with one lens - a 35mm prime - and no flash.
I like the 35mm focal length because it makes a great street-shooting lens with the 5D and it also becomes a "normal" lens on the 50D (or on any APS-C Canon SLR). The low light ability and the increased control of the DOF can make primes more versatile than zooms. The smaller size makes it easier to carry around as well.
I thought it might be useful if I posted some sample images. These 5 are all uncropped from the 5D. I added the "crop factor" rectangle of what/how I might have composed the shot had I used an APS-C camera (and if I did not change my shooting distance).