Starwood allows Elite Members to hand out certificates to it's employees as a way to say "Thank You" for great customer service. Each elite member gets 8 coupons. I know that Delta Airlines sends me 2 coupons a year to hand out. iahflyr can verify if United does the same. (my guess is they probably do. all major airlines copy each other!!) I don't know what the flight attendants get from receiving one, but I do know they are very grateful when I hand them one.
I haven't had the opportunity to try this at a SPG property, but a "Nice Suite Upgrade" would be nice!! Maybe upon my next check in, I introduce myself and slip them a "Coupon" before they give me my room assignment!!
P.S. - I hate Terms and Conditions!!
Here is the link, if anyone wants to print them out: