I'm with SB on this one - I'd buy something generally acceptable. Pay for them to go out for a meal for example. You don't know that they might have the thing you are considering, or not want/need one.
anngel - that IS a good idea. We used the sendacow site a couple of years ago, we sent funds for a family in Africa to buy two goats, on behalf of a religious person who didn't want a Christmas present.
Why must it be a religious gift? It depends on what you're thanking the person for, but Squarebear's suggestion of flowers seems good to me. They're acceptable in any situation.
(Waits for someone to say 'not if you're allergic to flowers'). :o)