As many others have said, a smile, a genuine thank you and general kindness will be more appreciated than any trinket or gift. At the beginning of my CP many, many moons ago I had more than one shift where I was only assigned to stand at the entrance of my merchandise location solely to greet people. I am a bit of an introvert (even more so then than now) and it was so disheartening to keep greeting guests with a friendly hello or welcome and have them look right past me as if I did not exist. The interactions I remember were the ones where guests returned my welcome with a smile or a nod or a hello back. When I go to WDW now I always try to return the kindness offered me by the cast members and engage cheerfully with their greetings and friendly questions. It is also always nice to hear that you made a difference in someone's trip or day so if a cast member does that for you - tell them!
Again, a million years ago on my CP I helped a mom and her child locate some characters they were looking for to get autographs. It was their last day at the parks, the dad and the older siblings had gone off to ride some rides but the youngest child had woken up that morning suddenly sad that he didn't get any autographs. They stopped at my merchandise cart to buy an autograph book and told me what they were up to. They had limited time so I looked up some character times for them (this was well before MDE) and gave them some tips on how they might want to navigate around to meet as many of them as possible. About 2 or 3 hours later they came back to the cart because he wanted to show me how many autographs he had collected and thank me. And then he shyly asked if I would sign his autograph book too so he could remember the Disney worker who helped him that day. It was one of the most magical moments of my entire CP and I still remember how happy it made me all these years later (even if I am not entirely sure any more if it was a little boy or a a little girl - pretty sure it was a little boy!) So if someone helps you, make sure they know. Even when you don't have to deal with a lot of outright rude guests as a CM, the bulk of your day is often dealing with guests who don't even really notice you are there so the moments when someone really sees you and acknowledges your work really stick with you.
What is the rule these days on even accepting gifts? When I was there we had to turn gifts/tips down 3 times and if offered again we could take it but had to report it to our managers.
Again, a million years ago on my CP I helped a mom and her child locate some characters they were looking for to get autographs. It was their last day at the parks, the dad and the older siblings had gone off to ride some rides but the youngest child had woken up that morning suddenly sad that he didn't get any autographs. They stopped at my merchandise cart to buy an autograph book and told me what they were up to. They had limited time so I looked up some character times for them (this was well before MDE) and gave them some tips on how they might want to navigate around to meet as many of them as possible. About 2 or 3 hours later they came back to the cart because he wanted to show me how many autographs he had collected and thank me. And then he shyly asked if I would sign his autograph book too so he could remember the Disney worker who helped him that day. It was one of the most magical moments of my entire CP and I still remember how happy it made me all these years later (even if I am not entirely sure any more if it was a little boy or a a little girl - pretty sure it was a little boy!) So if someone helps you, make sure they know. Even when you don't have to deal with a lot of outright rude guests as a CM, the bulk of your day is often dealing with guests who don't even really notice you are there so the moments when someone really sees you and acknowledges your work really stick with you.
What is the rule these days on even accepting gifts? When I was there we had to turn gifts/tips down 3 times and if offered again we could take it but had to report it to our managers.
but a glowing review, as mentioned above, somewhere their supervisor will see it would be much more helpful to them in a practical sense, I'm sure!
That's so gross! 
And its true, many wouldnt eat goodies from people they do know, let alone from strangers/guests.
