I found a new one last night. Hop on the back of a flat deck freight train and ride it around the country shooting at things. Probably best with a minigun or rocket launcher. There's quite a lot of sparsely populated area to wait through, so it's not great for killing records; but if your wanted level is high enough you'll have helicopters to play with. I expect I'll be having lots of fun doing this in the next little while, it's just annoying waiting for one to spawn in.
Hunger shouldn't be an issue, you can hop off briefly to grab something from the vending machines when the train slows down for a station. You may even be able to get an emergency escape vehicle parked on the train, I haven't tried yet.
The San Fierro tram would be another thing to try, but it's very difficult to survive for any length of time. Cops spawn in from all sides and pick your health down to nothing pretty quickly.
edit:
Did some more of this. I got the northwest quarter of the rail circut last time, and that seems to be the best part. The rest of it is mostly waiting for tunnels to end.
I still think it's more fun then the SF tram or riding the back of a Linerunner though. Too bad the rail network wasn't more integrated.
Oh, I tried the escape vehicle. They fall off after a minute or so.