Meghan's found yet another use for those stretchy rubber/silicone bracelets:
I stumbled upon this hack last year while visiting Disney World. The hotel we stayed at had a deal where you could buy a cup for a set price, and refill it at their cafeteria for free for the duration of the stay. I was trying to differentiate my cup from everyone else's, and I slid my Livestrong bracelet around the base of it. The silicone bracelet stretched to fit, didn't slip, and stayed on through our entire stay.
We have plenty of the bracelets around the house and since our trip I've started to use them to mark cups and water bottles. The bracelets are readily available for about a dollar each (depending on which charity they are supporting) but there are also companies that will custom-make bracelets for you. Order a whole set with your last name, or make a couple for each child if you want to easily spot a stray cup and know exactly who had it last.
Label sippy cups with electrical tape
Personalized rubber bracelets as wearable identification
Custom silicone bracelet as preschooler travel ID