There are several ways you can easily switch two cards. You can use a Double Lift, Palm off a card, perform a color change (there are several to chose from), top change, and the list goes on.
If you truly want to use a gimmick, the most visual one is called "WOW." You will see it all over youtube if you simple search wow magic trick.
I honestly suggest practicing the Double Lift. You can show a random card on the top of the deck. They will say it is wrong. Turn it face-down then place it on the table. Ask them to name their card and pretend to look for it through the deck. Then say are you sure that wasn't it? They will check and now it will be their card. Super easy to do with just a bit of practice (to make the move look as natural as possible.)