Knickers.

Commercial pairs (which I prefer) are available from Swrve, Swobo, Chrome, Under the Weather, Bicycle Fixation, among others, for $100 - $150. These are generally made from technical fabric so they're water and wind resistant, stretchy, comfy on a bike (you can wear padded underwear under them, I don't but I've heard you can), look good on the bicycle and off. It is the looking good off the bicycle that keeps me away from tights and shorts; I think people look silly wearing spandex/tights/bicycle shorts unless they're on the bicycle and racing.

Or, you can make your own knickers from any durable pair of pants. Military surplus stores are supposedly a great place to find wool pants to make into knickers but I've never managed to find anything that fit me very well.

Wear higher socks when it's colder (I go with knee high), and shorter socks when it's warmer.

I used to wear my street clothes on my bicycle and got tired of dealing with grease (no matter how far I rolled my pant legs up, grease still found its way onto them), not to mention most slacks, jeans and khakis are cotton which isn't fun to wear once it gets wet.