When I bought my Camry (2001 with 127k miles) the intent was to use it as a spare car and something for my girlfriend to drive to work...however, we love the way it drives (such a smooth drive and so quiet compared to my neon), now it's going to get 1-1.5k miles a month or more...over the mountains and such 2-3 times a month, so I want to make sure everything is shipshape.
Since buying the Camry with 127k miles, I've replaced the air filter, PCV valve, fuel filter, spark plugs, and spark plug wires. I got the timing belt inspected, mechanic said it'd been replaced before, so I don't need to worry about that for a while. All fluids have been checked and topped off if needed.
Is there anything else I should change out to ensure she keeps running for the long haul?
Should I get the timing checked? She gets an average of 30-34 MPG when being used on the highway (even when going over passes), which is better than my Neon most of the time

, so I'm not sure timing is needed.
I read around about O2 sensors, should I replace those, and if so, are they easy to replace?
Transmission fluid: I keep hearing different stories, people say to flush it, other people say to never EVER touch it because it just causes problems... transmission
seems fine, it just likes to jolt a bit sometimes when shifting at mid to high high RPMs (>3000).