Planning a first overseas trip with a strict checked-baggage limit

We're taking our first overseas trip with a strict checked-bag allowance. We don't want to pay extra baggage fees, but we also don't know what we'll need for different weather and activities. I'm thinking of packing clothes around a small matching color palette, wearing the bulkiest shoes in transit and leaving room for laundry. What did you pack on your first big trip that you never used, and what small thing did you miss?