I have a 2000 V6 Accord Coupe. It's purely stock with 215,000+ miles.
I have a couple of questions.

1. ​​​What can upgrade for better gas mileage.
2. What size Turbo can I bolt on without having to do major engine work.
3. Best cold air intake for my model car.

- also when on the highway I idle at 2500rpm going 75mph and 3000rpm going 80mph. Is it possible to go 2000rpm at 75mph and 2500rpm at 80mph?

 

1. Drive slower, maintaining a constant speed, increase tire pressure to cold max sidewall pressure, reduce weight, improve aerodynamics
2. Best to do some research of your own and find out what may be the best sizing for your 3.0. Tuning is going to be your biggest issue.
3. AEM is probably going to be the best quality for a reasonable price. But it won't really net you any gains.

Those rpms seem kind of high for the V6, but I don't know what the gearing is stock.
It's possible, but you would need to find a gearset that would work in your transmission or most likely have one made.
As it is, AFAIK, there are only two variants of the 3.0 4spd transmission, the Accord and the Ody. And the Ody will more likely have an even lower gear ratio set than the Accord.
Probably be better to swap in a 5 spd from the later Accord. MT or AT, but that will require work.

 

These questions are somewhat at odds, better performance vs better gas mileage.

For the better gas mileage: higher pressure in tires or lower rolling resistance tires, lighter wheels and rims for better unsprung weight, adjust alignment for lowest resistance, custom transmission gears for higher ratio to get less rpms at highway speed. Switch to manual and change gear ratio for lower RPMs.

However, keep in mind these changes almost all lead to worse performance and a crappier feel in the car: adjusting alignment for best mileage produces the least traction, higher pressure produce less traction and car feels bouncy, getting the lower rpm will lead to a more sluggish car at lower rpms. Only a wheel change might product improvements without consequences (except to your wallet).

For performance, this is just my opinion, but at 200,000+ miles I would not be doing a turbo unless you are rebuilding the engine. One, the engine is worn and the turbo will cause a lot of stress, and 2 - the transmissions are not really made for much more horsepower unless you also rebuild it. I either case, I predict breakage within a short time unless rebuilds are done to handle this increase in horsepower.

That being said, there are kits out there, getting this installed by yourself is a few thousand, having someone else do it is many more thousands of dollars.