Answer:
It mainly depends what kind of gag gift you want to get and what relationship you have with that person. There are many "inappropriate" gag gifts at Spencers that you can pick up like a pen, or a pack of gum that actually shocks you. Or a funny t-shirt. You know the person better than anyone on wikianswers so maybe just go into Spencers and see what reminds you of them.
Sorry I just reread your question and the above answer is for if you wanted to buy someone a gag gift. However, If you wanted to submit a gag gift idea into Spencers for them to cell then I would recommend calling their 1-800 number which should be on their web page and ask someone.