Hmmm...really good question. I guess I wouldn't expect to be repaid with more than a sincere thank you, but I'd certainly enjoy being thanked with some extra gesture - a gift basket, a gift card to a favorite store, flowers, etc. are always nice surprises.
If someone did something above and beyond and life-changing for me, I would try to express my gratitude first and foremost in words - via a sincere letter - and then throw in a gesture like a gift for the person, a gift certificate so that she or he could do something with his or her kids/family (dinner at a favorite restaurant, passes to a sporting event, etc.).
Personal experience? My parents provided free childcare to me when my oldest son was born and I was a single parent, and they welcomed us into their home. Because of their help, I never had to use transitional assistance or food stamps or anything like that and was able to take a job that turned into a great career. They then also watched my younger kids 3 days a week until they were in school. It's well over $100K in childcare that I would never be able to repay them for. Over the years, I've tried to show my gratitude in several ways - first, by being there - cheerfully - whenever needed, by thanking them publicly when toasting them at family events, and by giving them little surprise gifts like restaurant or movie gift cards.
I was a gestational carrier surrogate and although I was compensated, the couple I carried for was very generous above and beyond - a generous gift card to my favorite store for my birthday, hockey tickets for my husband and kids, a gift basket of treats for the family for the holidays, gift cards to restaurants, etc. Those were all nice surprises but not expected.
For your second question...luckily no one has done that for me (and I haven't done that to anyone else) but if someone who had helped me then felt the need to remind me of that after I thanked them, it would bug me but I would try to take the high road and cheerfully say something like "yes, and I sincerely appreciate that. How 'bout them Red Sox?"