If it's a type of exchange where that may happen, then neither side likely needs the money. It's best to get just give a card. I think cash is more appropriate for parents/grandparents/aunts giving money to their kids/grandkids/etc, and less appropriate between peers. If you feel the need to get your peer something, then get them something you think they may like. If you don't know what that is, then IMO you should stick with a card or nothing at all.

My family has been moving away from gift giving, in favor of spending time together instead. Show people you care for them all year long and don't buy in to these over-commercialized traditions.