Didn't you mention once that you had a pit at your place ?
How about doing minor servicing, just oil & filter changes and a basic check over on the vehicle.
You could end up with some extra work replacing simple stuff like engine belts, light globes and what not.
If you're just looking for a bit of extra cash in hand this might be alright for you.
But if you're looking at becoming a legitimate registered business it will be a whole different story though, then you have to have an ABN number, setup a business banking account and also some form of insurance for your own protection and probably other things I've forgotten....These aren't hard to get but just something to keep in mind.
Good Luck with whatever you choose to do though mate