i think you have to realize a few things here:
1. garages are a place for assured security and a place to do private car work.
2. parking your car outside for 8 hours a day (if you work) is going to land about as much dust/dirt as the eye can tell on your car as leaving it outside for another 8 hours at night.
3. a covered parking spot, if you aren't the type of person to need privacy when working on your car (although i do generally prefer it), is going to bring you the same benefits as a garage, minus what i said in #1.
4. people generally don't care about cars. when people see a good looking car, they appreciate it. washing it everyday or 5 times a week and waxing it every week is normally going to be something no one but yourself would notice.
5. if trees are unavoidable, as silly as it seems, buy a car cover. people might think it's dumb, but your tree sap problem isn't going to solve itself.
i've had a garage, no garage, and covered parking, and while i prefered having my garage, not having one is a worse thought than it really is in reality. my advice - give your car a good polish/wax, keep it clean on the consistent sunny days, avoid trees, and your car's finish will look better than 95% of the cars out there all the time (and that same 95% of cars are probably also un-garaged). there is really no reason to take it to a greater level unless that greater level is the only level that you will achieve satisfaction; and if that's the case, i say find a second hobby
