One bike to rule them all

I've gone for the one bike to do everything practical approach.

CycloX commuter type bike.
Load carrying;
Distance riding;
Kiddie transport (seat and/or trailer);
Commuting;
Winter riding;
Light trail riding;

Then individual frivolous bikes, totally focused on a single use*.

Fixed road bike
Fair weather road riding for short/med distance fun and fitness.

SS offroad bike
Off road riding for short/med distance fun and fitness.

(*TBH its difficult to travel far without gears where I live)

^ perfect!

Ed smallfurry has it - SS offroad = polo bike!

get yourself a frame that has a high BB, vert dropouts, deralier hanger, V brake mounts, get a pair of 29er wheels with a fixed cog for Polo and freewheel for commute.
This Is the future

Gary Fisher, surly and OnOne both sell frames that support these builds.