I've been thinking about this also, and the 'model' would be to set up something that can be amended slightly if required to suit each customer, then re-sell. Sorry, I'm going to be a bit vague (my chosen idea is a bit different to this but on the same 'model') but ... whether it be a bookkeeping business setup pack, hiring out something like a slushie machine, party kits (kids pirates, adults 'casino') or even an electrical generator or temp fencing - basically anything that you pay for once and get the use out of again and again. If it's a 'physical' thing might involve perhaps a once a week clean and perhaps delivery/setup on a weekend. But the idea is the same. Buy it once (and make sure you get something really good quality), price it well and use it over and over.