Yeah, a standard screw-on single-speed freewheel on a Sturmey Archer S3X hub will still allow you to change gears and will give you the option of coasting. Check BMX shops for a freewheel. They could probably get you one, installed, for $40-50.

Info on the hub on the bike I linked:
http://www.sturmey-archer.com/products/hubs/cid/3/id/47

You won't like most "fixies". They typically don't offer gearing options while riding and typically don't allow one to coast.