When we travel in and out of a place (like Madrid or Paris), we carry a very lightweight duffle bag with some extras and leave it at the hotel to be retrieved later. And since our hiking poles needed to be checked anyway, this worked fine. However, sometimes we don't fly in and out of the same place and so we carry the duffle bag (which weighs only ounces) in the backpack.
As far as clothing for "dress up," ever thing I need is also part of my Camino packlist. I have a pair of black suede Mephisto sandals (1 pound) that look dressy and are comfortable for walking around town. I have a light-weight skirt, an extra blouse/top, and the full-slip (that I sleep in. I have a Smartwool top (base layer) that is black and so I can wear it just as you would a sweater. A scarf and earrings complete my outfit.