The 2002 STS Owner's manual gives, as 100,000 mile service:
Change spark plugs (ACDelco #41-987 Platinum)
Change transaxle fluid - IF the car is used in severe service (taxi) or dusty or towing conditions.
That's it. Check the two serpentine belts, keep up on your oil/filter changes (7.5 qt. total, no more), and drive. When you do the plugs, do it COLD, no anti-seize, 13 ft/lb of torque, and take a hard look at the connector boots for tracking or cracks. The rear coil pack is a chore because the rear A.I.R. injection valve has to come off, and its metal feed pipe needs to be disconnected down behind the rear cylinder head. Not fun. IF you decide to change the transaxle fluid remember that you have a dry sump unit: there will be NO FLUID on the dipstick with the engine off. The engine must be running, warm, in P, to properly check the fluid. Fill to halfway up the dipstick marks. If you can, use the new synthetic DEXRON-VI or equivalent. It is 100% compatible with old DEXRON-III.