I really enjoy taking friends flying. I feel that it is part of a GA/Country-wide recruitment drive into GA, which will pay dividends for all of us in the long-run. Frankly, many clubs and schools don't do such a good job at advertising GA (IMHO).
If someone offers to make a donation, I'll happily accept and am happy with a fiver or fifty.
I've had some surprise favours in return, like the time I had major problems with my car, and the mechanic who I'd taken flying a year before had never forgotten it, and did the repair work for nothing.