There are plenty of kinds of surprise that can be wonderful in the right context. Perhaps easiest is when someone tells you they want/would like something, you keep an eye out for opportunities, and surprise them when it comes around. (My wife wanted to see a comedian. I subscribed to their newsletter, and then got (refundable) tickets when they were performing near us, years later.) There can also be things that you are confident someone else will like, which they don’t know exist. (I bought a friend a battery operated pruner after seeing him struggle to keep up with bushes.) Or small things that someone likes (flowers, chocolate, breakfast in bed, depends a lot on what they are into).

The guitar repair was the worst kind of surprise: something irreversible, which the surprisee clearly knew about, and has chosen not to do at every single opportunity - while the surpriser doesn’t know why they are choosing not to. That’s not respectful, which is the absolute minimum requirement for a surprise to work.