How could it have been a surprise if it hadn't been kept secret?

If you have never done something secret for someone, I recommend it highly. One year Mister and I and a friend chipped in and had a seamstress make a little set of reenacting clothes for another friend's little girl. I had it shipped anonymously. It was all a big secret, and great fun. Of course I had to admit to it eventually, mostly because I am a lousy liar!

I also take great delight in leaving treats on people's desks in my office - anonymously.

I don't do this to win favors. I do it because I want to. It makes them happy, it makes me happy. Try it sometime.