It's always been my experience that last thing I have time to do prior to opening is worry about individual gifts for the cast. Also, it's important to remember that if you're going to give gifts to the cast, you should also include the crew, musicians, etc... as to not show favor to one element of the company.
A simple and elegant alternative that I've used in the past is to provide something like a fruit/veggie tray in the green room for the entire company to enjoy. This shows the company you're thankful for their work, includes everyone involved, and also requires very little time and effort on your part.