I love my single-speed for recovery rides! I geared it 42t x 16t, so it encourages me to work on my spin.
Did it cheap since I convered my old ('77) MASI Gran Criterium. Same wheels, just took off the Regina 6-Speed and threaded on a BMX (3/32) 16t freewheel. Took off the big chainring, brought the 42t to the outside, removed derailluers, shortened the Regina chain, cut off bars and put on cyclo-cross brake levers.
So, a few bucks for the BMX freewheel and cyclo-cross brake levers, otherwise (mostly!) free.
Rick / OCRR