If you can sneak your shoes
If you can sneak your shoes off for a bit you could bring a tennis ball or a golf ball and give yourself a nice foot massage while your sitting. Rolling your foot around on a ball with just a bit of pressure can help loosen up your feet a lot. If you don't think you could get away with taking your shoes off for a bit, I dunno, wiggle your toes? Haha, I guess it could do something.