I agree on clothing. With synthetic shorts and T-shirt, which dry really fast, I don't feel the need for a change of clothing until I get beyond 3 or 4 days. The closest I come to "backup" clothing is the windbreaker I always carry; it's only "backup" in the sense that my rain jacket could probably do duty as a windbreaker - but sometimes, when it's not-quite-cool but windy, the rain jacket is too hot but the windbreaker is just right. So, I don't consider it backup.

Beyond that windbreaker, I carry a change of socks and, if I'm going out more than 2 days, a spare set of boxer briefs. Over 4 days, I might add a second pair of socks and a spare T-shirt and shorts in hot, sweaty weather.