I'm curious, why couldn't you just do a change request and have the guest accept it ? This only takes a few minutes, at most, from both guest and host.
As for the extras, depending on our feeling about the guest, mostly from the way they inquire / request the booking, communication, etc., we will leave a bottle of local wine. If it is for special occasion, ex: they celebrate an anniversary, I might even engrave the bottle based on info they disclosed before their arrival. Nearly all never mentioned it in their public comment/review... A somewhat good thing, otherwise others might expect it. However, I few asked if I could do few wine / champagne glasses or wood signs for them. They paid for them.
We had one guest who asked if we could do them breakfast on their check out ... vegan. I'm an omnivore with strong carnivore tendancy. We were thinking of starting to offer some meals, so what the heck. I said : sure, I'll give it a try. I was thinking of doing some fancy breakfast. All they wanted was vegan pancakes for her, eggs & bacon for him. We added fruit juice, fruit salad and such. Offered free given it was a trial. They loved the breakfast. They have returned three more times since. Each time I offered them the check out breakfast. We are likely to make it an option starting in 2013 (+$).
All this to say, either :
1) include the cost (no direct profits) in your rates;
2) do it only for guests you feel will appreciate the extra touch, not expecting a better review;
3) offer extras as optional items they can order (and pay) before arriving.