New serpentine belt and tensioner: Gates K060551 belt ($16.96) and Dayco 89625 tensioner ($112.79), both from Rock Auto. The original belt still looked fine and I didn't see any issues, but after 18 years and 200k miles, Gemini and I figured we should go ahead and replace it. Installation was pretty straightforward.
I replaced the engine air filter with a Bosch 5154WS that cost $5.43 from Rock Auto! Since it's so cheap maybe I should be changing it more often. The manual specifies the engine air filter to be replaced every 30k miles, and the last time this was done was nearly three years ago at 174,484 miles.