Fixed gear with more than one ratio - talk to your friends at Sturmey Archer about their S3X hub - dingle cog which I believe (and may be wrong) can't be changed on the move - flip flop hub (which shouldn't be changed on the move) - go to SS. I think thems the options.

The dingle cog has to be worth a try, especially if you have a good quality quick release on the back (it HAS to be good, otherwise use nuts and a spanner). I've gone the S3X route, but I'm lazy (don't want to be mucking about with the rear wheel).