^ The themed t-shirt thing may not be acceptable at some schools. We (read: the entire greek system) were absolutely forbidden to wear or display any letters during work week. (We actually had one chapter try to slap another chapter with a rush infraction, because a member of the other sorority who attended a different college went for a jog, was wearing a sorority t-shirt, and happened to jog past our school.
)
Anyway, suggestions ... Get some nice, loud, silly music to listen to while doing boring things like making decorations. (You could even do themes with that - play '80s music one day, for example.) (Warning: This can backfire. To this day, I cringe when I hear "Cheeseburger in Paradise".)
We had some fun while practicing our conversation skills, too. We had workshops where two sisters were pulled out and one of them played the role of a PNM with an ... unusual ... characteristic, and the other was the sister who picked her up at the door. The rest of the chapter would watch, and the rush chair would critique the convo (e.g. "What else could Annie have done to get Shy Violet to open up and give more than one-word answers?" or "How could Susie have steered Franny the Frat Rat away from the topic of which fraternities have the hottest guys?"). Well, I got to be "Promiscuous Pam", the campus slut. Which is so far from my personality that we were pretty much all in stitches.
One year, we actually had initiation during work week. We had one new member who'd pledged in late spring, too late for her to be eligible for initiation before the semester ended. She started her NM program in late spring and finished up over the summer, and we initiated her during work week. It served as a reminder of who we are and what we stand for - something you can lose sight of when the rush chair is barking orders and you're sitting there with glue in one hand and glitter in the other hand (and in your hair, and all over your clothes) while listening to "Cheeseburger in Paradise" for the quadrillionth time.