Generally speaking you shouldn't have to do much hardware maintenance, however there are several tings you will need to keep track of on a schedule.
These are the items you should put on a calendar:
For Computers that have fans - Every 3 months blow out the fans with a compressed air (purchase compressed air from your local computer store.
UPS Battery Backup Devices - Replace once a year.
Credit Card Readers - As needed clean out the readers by wrapping a card in plastic wrap and swiping back and forth.