You wonder why (on cars without standard A/C) the A/C option costs about $1000, when you can buy a portable room A/C for $140 at Walmart.
mostly becaue a cars ac system is alot more complicated. which means, new mouning brackets, new holes to make in correct locations, new hoses to route wires to run all in compact places. a home AC unit is a box with all the parts inches away from each other and thier incredebly easy to work on and as long as all the parts are there and its in the box its good enough. you dont have to worry about runnign wires 5 feet away, hoses to keep from melting etc...
and some cars have to have the coolant flow thru the heater core otherwise theres no coolant flowing thu the engine.
If the car requires water to flow through the heater core (I've not seen one yet) then the water line can be rerouted during the summer months. My 96 Tahoe had a vacuum operated valve inline that would divert coolant back to the engine without going through the heater core unless the selector switch is set to hot. One of these valves would solve that problem, but would cost more money. An electric/vacuum switch could be used to allow a dash switch to open/close the valve on demand.
1) As another person said, buy those window screens, and put them in your windows whenever you stop. And put some thought into which ones you get (the ones that look like reflective silver are generally more effective then the ones that just block light). That will make a HUGE difference in how hot the car warms up when sitting in the sun.
2) And if you have any easy option to do so (garden hose? car wash?), wet down the car before taking off. Wetting down the car will have two very useful effects on heat. First off, the water itself will cool down the metal of the car a noticeable amount. So you get an immediate cooling right there. And secondly, some of the water that remains on the car will dry/evaporate as you drive down the road. And that evaporation process will also cool the car/cabin.