Really depends on how deep we're getting here. Some things off the top of my head...
-New thermal pads / paste for the coolers.
-Replace clock battery (or remove if you're trying to preserve it).
-Check HDD SMART status, possibly also make a working backup.
-Dust it.
-Visual inspection of non-solid capacitors. Maybe replace them, it could have been made during the time of the capacitor plague...