What maintenance (preventative or otherwise) would you recommend I do while I'm in there?
What a question! That really depends on how long you have owned the bike and what you have done, as well as what your inclinations are. For a new, used bike, a complete fluid change/flush, (oil, coolant, brakes, forks) is mandatory. I'd toss in change the filters too. From there I'd go to cleaning the caliper pistons and inspecting the pads. Depending on mileage and age, spark plugs, adjust cables, check battery and electrical connections (grounds) for corrosion and/or signs of overheating. Your '04 is 15 years old, so inspection of all rubber components (read hoses) is due, and some might have to be replaced. I would be inclined to change the thermostat, too. One of the guys on another website is quite vocal about how compulsive he is (as a mechanic) and he would go over your bike from stem to stern, checking torques, and everything on the bike. I know he would loosen the fork pinch bolts, retorque them, loosen the front axle and bounce, tighten the pinch bolts, check the steering head, and work his way back on the bike. Clean and lube the chain, adjust chain tension. Check tires, air pressure, etc. Depending on the age of the battery, or if you can determine that, I might put in a new one - having a dead battery on the road is inconvenient at best, and downright expensive if done in a small town in the back of beyond. I routinely change batteries in my cars and bikes after 5 years of service if they have not already failed, and sooner if I'm going on a trip. Our car battery died a few years back in a small town in eastern Ohio and a replacement at the local NAPA store cost me twice ($150) what I would have paid back home from my local Battery purveyor. And it took about two and a half ours out of our one day trip dealing with finding someone w/ jumper cables and that NAPA store.