I'll add to the folks suggesting the Red Oxx Mini Boss - I've done a full seven days mixed business and personal travel with just that one bag. It easily fits under the seat on all the domestic US mainline aircraft I've tried, very comfortable to carry and looks good enough to pass in a business meeting in most cases. Should be no problem to do 2-3 nights without even needing to do much strategic packing.
Tom Bihn makes some great choices in this realm as well but they are a little more expensive and I don't have hands on experience with them.