prices need to be kept reasonable, but i have no issue with a"donations appreciated" line.
courses that are scenic and travel by nice natural areas and historic places are nice.
the most successful charity rides in my area tend to have 4 course options. A 10 mile or so family friendly loop. A 25 to 30 mile fairly easy loop. A moderately difficult 100k. And a full century.
I'd add that it is generally accepted to charge different rates for the different lengths because of the sag expenses.