To be honest, my first mixer is this Saturday, so I have ZERO experience with this, but I found a fun website through Delta Chi that is mostly ideas (and you said you had a theme) but I think that a lot of the activities could be tailored or built on. What is the theme you have? Maybe that'll help some of us get the creative thoughts going!
Either way, it's a fun site, and you should check it out!
http://www.triangle.org/docs/delta_chi_social_101.pdf