Some progressive offices allow for jeans, but sticking to dress slacks and chino pants (commonly known as khakis) will ensure you're well within your dress code standards. A pair of khakis can be a particularly versatile tool in a business casual outfit, as they can go well with more formal and more casual shirts alike.
While it depends on how adventurous your office is, try to stick to plain or neutral-colored pants like black, grey, tan, or dark-toned colors. Avoid bright colored bottoms (neon yellow isn't always the most professional, unfortunately). Always include a belt in similarly neutral tones.