What to wear on date?

My boyfriend and I will be going out to celebrate our six month anniversary in a little under a week now. Thing is, he won't tell me what we're doing or where we're going (he says it's a secret, haha), so I have no idea what to wear. I don't want to be underdressed or overdressed, so what would be a good outfit for the occasion that is not too dressy, but not too casual either? I need something that can be dressed up or dressed down, if that's possible. Any suggestions/help would be greatly appreciated! :)