I agree clean it up pretty (tracks, windows, controls, arm rests, seat), full tank diesel, and throw in a gift certificate for steak house and maybe a bottle of whiskey/case beer if he likes that sort of thing. Rental rate for 8hr day would easily be $700 for cutter and machine. Operator/machine rates are usually $150/hr. I believe $50 an hour plus fuel is a good ball park of machine/cutter operating cost per hour till the machine is paid for.
Also since I assume you are close neighbors as there is a level of trust with the machine if he mentions he is going to be doing xyz the next day etc just show up and give him a hand. I do all kinds of "free" machine work for my neighbors and the best way they can pay me back is picking up limbs and mowing the yard or hauling my trash to the dump. Really anything that shortens my day is a welcome thanks. I hate taking money from neighbors that I know, but a basket of eggs or a cake, that is the stuff the keeps me helping.