This is also a great time for you to learn the score- if you're not a great sight reader, or if you have problems reading music in general this is an oppurtunity to follow the score while the singers are moving slowly through it & the Music Director is in the room.
And yes, this is only exciting for an hour or so... But if you bring your lap top, you'll be able to work out props lists, scene shifts... etc... once you're bored of the music.
Enjoy!