One of my best sellers at conventions is sketchbook commissions. Everybody treats these commissions differently, but in general they are simple to somewhat complex sketches. I’ve also seen sketchbook commissions inked, fully colored with markers, or even painted! They can be extremely complex, or simple stick figures; it all depends on the artist and what they’re willing to do.
Con-goers often provide their own sketchbooks for these commissions and treat them as collections. They will go to different artists around the convention, getting artwork from each of their favorites, slowly filling the books. Some people choose to get random work, some people theme their books. I often get book that have a character description in the front and everything that is drawn within has to be based on that. I even got one book that had a random quote on each page and the artist could choose whichever quote they liked best to inspire them.
They’re great! They’re really nice, personalized collections for the people who purchase the sketches, they are (generally) a simple and fast way for artists to make money, and they are great promotions/advertisement for artists too!
I have a stack of about 8 sketchbooks that I need to complete by tomorrow soooo… I really ought to get to that now.