Either have a choice of two wheels and decide which to fit before the ride, or use a flip flop hub. Anything more complex takes away half the point of riding single speed or fixed.

Single speed requires less technique and concentration than fixed. People who like riding fixed enjoy the challenge and the feeling of oneness with the bike. Each has its pros and cons and the choice will depend on your needs and preferences.

If you have fixed on one side of a flip flop hub, and free on the other, you will start off thinking that you will change regularly, but in the end you will find you use one side all the time.

If you put two fixed, or two free, cogs on a flip flop, you will probably find the same.

If you prefer freewheel and want a little more versatility than single speed, consider hub gears. Modern ones offer 8 or so gears and cost an arm and a leg, but the traditional Sturmey Archer 3 speed offers three widely spaced ratios. The middle one is direct drive (no additional friction losses), the top one is +33%, and the bottom one is -25%. As my dear old dad once explained it to me: one gear for up hill, one gear for the flat, one gear for downhill.

Or you could do some serious searching and find ether a close ratio 3 speed or — yes, really — a fixed 3 speed.