Pack Flexible Clothing
It's clichéd but true: When in doubt, dress in layers. Plan a few key outfits, and then add or take away, depending on the weather and the event. You don't need a different outfit for every stop on your itinerary ... just bring clothes that can do double-duty for both nice and informal occasions. Keep in mind, though, that many religious sites expect modest dress, so set the bar there if that's a part of your travel plans.
The same rules hold true for shoes, too. Shoes are a serious space-killer, so unless you have a good reason to bring more, stick to one or two pairs, and make sure you'll be comfortable walking in them all day.