I think it sounds like you are pretty good friends. They obviously trust you to stay in their unit and are comfy asking you to mow the lawn etc. It sounds like they are just doing a favour without expecting anything in return. So I think ask they what reds they like so you know you will get something they will enjiy and I would add that to a box of chocolates and a card just telling them you had a good time and really appreciate it etc and maybe judt sayigng you owe them a favour.