I agree with wendywhite. A toned-down version of a Cosby sweater (a fun pattern or colors, but no over-the-top embroidering or anything) and dark jeans would be appropriate (for both male or female outfits). If the weather is warmer, a short-sleeve “bowling shirt” and jeans (for a man) or short-sleeve neon top and jeans (for a woman). Either way, stick with jeans, sleeved tops, and closed toed shoes. You can “professionalize” all three of these options by wearing a collared shirt underneath the sweater or finding a collared, pressed version of the bowling shirt/neon top.
And this is not a silly question at all. I interned twice with a Big 4 firm and both summers we had an intern “casual” baseball game. It was always supposed to just be us interns and the HR staff, but a few partners “randomly” dropped by during both events (as they did at all of our events, I later realized). The first summer, I wore sandals and shorts and had to meet the office managing partner. Everyone else was wearing jeans and polos or official baseball jerseys. No one chastised me, but trust me, it's better to be on the safe side.
As a helpful note too for casual Fridays: at least at my Big 4 office (in the Midwest – things might be more or less lax where you are), “casual” meant swapping your work trousers for dark jeans. This meant wearing your normal collared, button down, sleeved shirt with bootcut dark jeans (no capris, skinny jeans, etc for the ladies) and closed toed shoes. Most people actually skipped casual Friday because wearing heavy denim and dress shoes is really uncomfortable in summer heat!
Sorry for the text wall. Hope this helped.
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