Here is a list:
Some will require extra equipment, some other will require specific license or insurance coverage.
Power Sweeping: extra equipment (not cheap), Big liability insurance, Expensive workers comp, Expensive auto insurance, only night shifts.
It is a very tough market with a lot more low balling then PW field.
Parking Lot Striping: In some states/counties a paving license or ZIP permit is required. Overall its easy money (for re-striping). For new striping, you need experience and you need to know how to read blueprints.
Most common paint is water based, sprayed with airless spray machine. Best machine on the market is Graco which you can rent as a first timer. Oil based paint is used on roads, streets and hwys, not that much on parking lots.
Porter Service: Good money. You don't need any licenses or permits or experience. You need: A porter guy, a trash bucket, trash bags, broom, mop and a pick-up tool. Although you need to establish a good and long term relationship with property managers which is the hardest part of all. You also need to keep a very close track on your porter guys.
Bulk Trash Removal: Good money, no license required. You need a pick-up or bigger truck for that purpose.
Pothole repair: cold asphalt, concrete. Good and easy add on.
Light maintenance: small repairs, Christmas decoration and etc.
Asphalt Seal Coating
Concrete repair
Paving: seal coating, repair, re-sanding
and etc..