Experiences are more valuable than things. Especially for people who are as cosmopolitan as these people seem to be. We all have things we have wanted to do in our home town, that we just haven't gotten to. Doing them with an out-of-town guest is the perfect excuse to create a memory with someone, and to experience something you have been wanting to do. So pose them the question when you arrive: "What would you like to do here in Geneva that you have not done yet?" Then go do it with them, and make it your treat. The experience doesn't need to be stored. It doesn't burden them with an object that they now need to take responsibility for. And it is something you can remember and share with a few words, when writing to them years from now. "Remember the coffee shop on Lake Como? That's a happy memory for me, too..."