Jon asked a question on the rules thread about whether civilian service counted (it does), and that got me thinking that actually, post-military civilian service is a really good scenario for extending the service life of a piece of hardware, since surplus military kit is often cheap for the amount of performance it provides, and the fact that it's military characteristics are obsolete is usually irrelevant.
The first idea I had was for a Lysander crop duster, but a quick Google search shows that not only was it done for real (in Canada), but someone'salready made a lovely model of one and it's been found by Dizzyfugu over on whatif:
Westland Lysander Version agricole / Agricultural version 1949 by
Franclab, on Flickr
No reason you couldn't do a what if version though: my concept was slightly different anyway.
You could do lots of other types as crop sprayers. One amusing idea I had was a De Havilland Mosquito converted to spray pesticides as part of a UN anti-malaria programme: a Mosquito killing mosquitoes...
Some other possible civilian uses of ex-military aircraft:
General passenger/freight transport
High speed courier
Survey (certainly photographic and possibly other sensors too)
Rescue
Water bomber
Firewatch
Movie camera platform
Fishery control (some agencies are civilian, some are military)
Coastguard
Pollution control (detergent spraying)
Warbird (air shows, joyrides, sponsored publicity, movie appearences etc.. I's say it's "in service" if it's still earning it's keep)
Tanks and AFVs:
Rough terrain transport
Bad weather transport
Mobile platform for construction/industrial equipment
Snow clearance (dozer blades or jet engines)
Rescue
Law enforcement ( e.g. SWAT vehicles)
Secure personal transport ("armoured limo"for really rough areas)
Firefighting (airfield, forestry, oil wells etc..)
Ships, boats and watercraft:
Eccentric private yacht
Survey vessel
Science vessel (Cousteau's
Calypso was an ex-minesweeper)
Ferry (landing craft, amphibians, hovercraft)
Rescue
Law enforcement
Fishery control