Since I asked y'all for input, I thought I'd show you what I ended up doing for my co-workers.
I also made chocolate chip cookie dough cupcakes and chocolate chip cookies, but somehow I managed to get no pictures of those. Whoops.
Goodie bags (Love the rainbow colors!!):
Thank you banner made by the kids during one of our story times and decorated with pictures of the kids and their rainbow (!!) crafts:
The number of M&Ms equals the number of reading cards we had turned in. I had people guess the number, winner got the candy and jar. This was BY FAR people's favorite thing. They got really into the competition and had tons of fun with it. The guy who won used a long, complicated math formula (no joke) to average the volume of a cylindrical prism or some such nonsense. I was pretty impressed!
This was a fruit salad our manager made. I love that it's in a pitcher, such a pretty presentation!