I just commute in my work clothes, which is usually cargo shorts and polo/bowling kind of shirt. Fortunately I don't sweat much at all, so that's not an issue.

When I do feel the need to wear long pants I use Champion G9 sport pants from Target. They're comfortable and keep my legs warm and just wear them over the shorts. I usually plan around rain but occasionally get caught in it and just deal with drying out at work.

Not much you can do if you don't want to wear rain pants. Just keep some extra clothes at work in case you get soaked.