What does a CouchSurfing host expect from their guests?

The first thing to know about Couchsurfing is that it is completely free. You are not required to "pay" for your stay offering a dinner, bringing a gift, etc. The only rule that applies is to behave politely. You are not even required to spend some time with your host, even though in some cases this could fall in the "unpolite behaviour".

This said, the answer to your question is very subjective and varies depending on who is going to be your host. I divide my answer in two paragraphs:

Gifts

During my personal experience in the community (I'm a CS member since 2009 and I hosted more than 50 travelers) I met couchsurfers who brought simple gifts (e.g. a box of chocolates, a can of beer, objects supposed to bring you good luck, etc.), sometimes people who prepared a whole dinner for me in my house or even a person who invited me a dinner in a restaurant. Several times the guest simply spent some time with me, chatting and having a drink, without offering anything (that was ok too, of course). The rule of thumb, in my opinion, is to read carefully the interests of the person who is going to host you and to act in consequence: if a person is very interested in food you could bring something to eat from your country that is impossible or difficult to find in the country of your host. The same applies for photography, books, music, etc. Remember: since you are not required to bring a gift, every gift will be well accepted!

Visiting the city

I usually like to spend some time with my guests and to show them the city. Other surfers I met just give you the key of your home and they behave more like a free hostel. Anyway people usually write down on their profiles if they are willing to visit the city with their guests, so just check the profiles before sending request.

It really depends on the host.

I try to help out with dishes and buy & prepare the food, or bring a postcard from your city and leave a nice note on the fridge before you leave. Some hosts expect you to hang out with them. I went to a bar with a local band playing in Vancouver I wouldn't have visited otherwise, and it was actually a nice evening.

Other host are hardly at home, so you can't really do much, but in this case I try to leave the place cleaner than it was before I visited. Clean the kitchen, bathroom and "public" areas if possible or maybe wash the windows.

My wife and I have hosted many people from many countries. We give them a nice bed, their own room, usually entertain them and, almost always, breakfast as well. We are experiencing an increasing number of guests who don't even make a gesture of thanks. When we joined we understood that there was supposed to be some kind of reciprocity. I always offer to help in some meaningful way when I stay with a host. It's common courtesy.