I think one of the toughest (but strangely popular) Jeopardy categories is Before and After, which combines elements from two items listed in the question. (For example, if it involves people's names, then the answer would combine two people's names. This is really tricky to explain - that's why they're tough ones to answer!)
Some other good categories are pop culture/sports, history, Rhyme Time, Begins with (fill in the letter), and (forgive me, I forgot what this is called) when the answer is a word or phrase buried within the question itself. Those are always fun to try to pick out!
For Potpourri questions, you could do random trivia on people, places, or things. If you need specifics, I'd recommend checking out this site, called Cool Quiz! (formally known as Uselessknowledge.com):
http://www.coolquiz.com/trivia/
For more ideas, you could check out Jeopardy's website:
http://www.sonypictures.com/tv/shows/jeo...
Disney
Geography
Quotes
Books and Authors
Logos
http://www.slate.com/articles/arts/culturebox/2011/02/ill_take_jeopardy_trivia_for_200_alex.html