Unlike with work exchange arrangements, there isn't the built-in structure of "I work for my room and board" that may alleviate any feelings of guilt or intrusiveness towards your host.
I find it absolutely impossible to remain guilt-free while a [potential] host opens their home to me, and especially if he/she/they are super welcoming upon my arrival. Here are a few things I've done to repay them under the "free" CouchSurfing system:
1. Bring a gift from your last stop
This one is easy enough! Depending on your budget and the hosts' interests, you can pick up a gift food item at the supermarket in the country before your host's, or you can purchase hand crafts, small home decorations, etc. Most supermarkets I've been to have had a "gourmet gift" section.
2. Make them dinner with ingredients you purchased yourself... And clean up afterwards!
Again, if you're not super loaded, this can be made with super cheap ingredients if you possess the ability to transform them into something tasty!
The nice thing about this one is that you can have a nice dinner with them as well. Depending on your guilt level, you may do this more than once. During a week in Italy, I cooked for my hosts every day! Remember to be mindful of allergies/dietary restrictions, and gauge their reaction to your cooking. When in doubt, cook just enough for everyone and low-pressure invite them to help themselves to the meal you prepared, rather than sit them down and tell them you are making dinner.
3. Help them around the house if you see them cleaning, hanging laundry, etc., but only if they don't mind!
Some people will legit not want you to touch their clean laundry (I say that because I am one of those people), but you can offer an, "Are you sure? I'd love to help!" if they refuse the first time.
4. Replace their toilet paper if possible
I go through paper towels like nobody's business, because to me they are sterile surfaces that can be used for any situation. Knowing that about myself, I'll buy replacement paper towels at the store, and while I'm there, I'll throw in a pack of toilet paper. If my budget is tight and I can only afford the quarter-ply (lol, is that even a thing?), I'll use it myself rather than use up my hosts' luxurious 7-ply.
5. Minimize your impact as much as possible
This should go without saying, but you don't want to be the sloppy roommate, the guest who runs up utility bills to crazy heights, or the person who steals all the bandwidth (for those with limited internet plans). Sure, you will need to take a shower (for your sake and your hosts') but don't forget to clean up AFTER you clean up.
6. Write them a great reference
But only if it's the truth! A lot of other travelers will be counting on your honest opinion!
7. Give the people what they want
Within reason! Remember, hosts each have a different motivation for hosting. Some may do it for company, and if that's the case, make sure to spend some time with them! Talk to them about their interests, share a few of your own. Exchange little things about your cultures! If you're social and like to meet other people, go out with them and meet their friends!
Some do it for karma. They may be planning a trip in the near or distant future, and they need either some spiritual kudos or actual references on CouchSurfing. Again, if they are great hosts, it should be no problem writing something truthful.
Some do it to learn more about other cultures. In that case, be a great ambassador for your country! Introduce them to beloved dishes, talk about the different customs you are used to, share with them a few words in your native tongue (even among English-speakers, there are different expressions/slang words depending on both the country and the region), and have a conversation with them!
In addition to all of these, some hosts may want to show you what their country is like, and might take the initiative in planning some fun outings for the two of you to do together! When friends of mine come to visit my neck of the woods, I already have a list of what we can do, and I both know and hope that they will enjoy it. A good host, however, knows that a guest will have his/her own plans for the area and will not force their plans on you every hour of every day.
Above all else... Be safe!! A roof over your head doesn't mean that unsafe situations should be tolerated. Some hosts are not emotionally or mentally ready to host a guest, so in some cases you will either need to A) bow out secretly, B) bow out politely, C) make yourself scarce for most of the trip.
Happy surfing!