I am all about thrift stores! I'd try to hunt up a pair of khaki slacks, since it's summer black would probably be over-doing it. A nice, light button-down blouse to go with it would work too. To avoid wrinkles on the way there, you could always wear a t-shirt in the car, then find a corner in the parking lot and change your shirt before you get out of your car. I've done it lots of times! My girlfriend (pretty girly) wears slacks and either a light blouse or pull-over "nice" shirt to work, often with her nice sandals. I've got it lucky, I think. I can come to work in a bathrobe if i wanted to! hehehe I'm in a t-shirt, jeans and converse sneakers right now. I love it!

Good luck with everything, though! I'm sure whatever you decide to wear will be fine.