I carry an Integral Designs siltarp/poncho in my shoulder bag as EDC. It might look a bit wierd walking around down town with an oversized poncho on but it covers any pack I might have and I'll take strange looks over being wet any day.

Doubles as a shelter...very multi-purpose. If I'm in the wilderness I carry ultralight tent pegs along with it instead of a tent.

I've done the mini-umbrella in the past but the poncho is about the same size stuffed into it's sack and way more effective