Gratis. Free. Pro Bono.
Some things I'd never dare to charge for. I'd also do some from the other list, if the situation calls for it and I can afford it! Conversely, I might charge for things in this list if that'd feel more fair.
Lifetime office hours, if you took a course/workshop I taught
Connections between good peeps doing good things
Inter-generational mentorship
(Un)solicited advice of varying quality
Perform/volunteer at a cool/impactful event
Advisory positions for impact organizations
Be a guest lecturer for fellow professors
Cook/send soup if I learn you're sick
Other things, just ask!