Aw, you are so sweet to already be thinking about those Cast Members that make such a huge difference in making our vacations run smoothly! I always make an effort to greet the Mousekeepers and let them know I appreciate the wonderful job they do.
But you can also bring plain envelopes that you write "Mousekeeping" on so that they know it's for them. If you have kids with you they might enjoy decorating those envelopes with stickers and markers.
It's a great time to use this as a lesson for our Children about tipping people that make our lives easier.
If you forget envelopes...em, like my family often does... there is nothing wrong with leaving the tip where the Mousekeeper can see it and and a note on the Resort's stationary that says a simple 'Thank you'.
Thank you for your great question...I hope that you have a Magical vacation!