Not too sure if this would be considered an alternative bike or not.
I just finished installing a NOS Sturmey Archer 5 speed geared hub on my mis 90s Kona cruiser, the Hunuhumu-Nukunuku-A-Pua'a. This is the S5-2 Alloy hub and other than the additional control cables and handlebar mounted shifter controls for the hub it looks pretty much stock. Much more versatile though as it now has a gear range from about 37" to 85". I classify it as a Frankenbike for now.
Remember there are lots of IGH units available out there, both used and new, and they can add additional capabilities to an older SS bike without disrupting the looks of it.
For the no control cable purists Sturmey Archer has announced a new 2 speed kickback hub that is supposed to be available this year too. Replace your old worn out Bendix kickback hub or add a lower gear to your old single speed cruiser.