I would go out tonight, but I haven't got a stitch to wear.

The end of the fiscal year was 2 weeks ago, and since then, my job has been yawnsville. Not that I'm complaining- I like my job, I like the people, I like the fact that it's stress-free and I can talk to my boss about things like my top 5 Smiths songs, or Stuff on my cat.
Anyway, the end of the fiscal year apparently means party time! Excellent! So this Friday, my office heads up to Baker Field (which is practically in Siberia at the northern tip of Manhattan), for softball, kickball, and a catered BBQ. Sweet. The question is, what do I wear? I work out on a regular basis, so I have gym shorts, tee shirts, tank tops, whathaveyou. But I can't imagine some of my co-workers in Underarmour (nor do I want to), so I'm stuck. My boss (of Smiths/cat fame) suggested that I "come more formal (whatever that means), and pack a bag if you get sweaty." IF I get sweaty?? We're playing sports in the 90 degree weather- I'll be a drowned rat upon arrival. I really only own one pair of non-gym shorts (I'm not a fan of shorts)- and they are red. So what do I wear- the red shorts and a tee shirt? Or do I just show up in my gym clothes? Is it unprofessional to wear a sports bra to a work event?