I'll be traveling to Split 5/31-6/2 and Brach/Bol from 6/2-6/4. Does anyone know what the weather is typically like? Are things usually crowded? I'd also love any travel tips and must see/do's!
May is traditionally Shoulder Season, but that late in May, things will be picking up. Split is a pretty large city, and the old section is pretty large. Of course, there is the Palace itself, which is fun. There is also a super Archeological Museum, which I recommend. If you feel the need for a day trip, consider Trogir by boat. The Palace is active day and night, and sometimes stuff in the evening is as fun or more than the day. They often have musical groups with the Croatian male quartet style called "klapa" in the main square, with lights. Very fun. Depending on your idea of expense, it can be somewhat expensive inside the palace, but there are plenty of restaurants.
I would caution against driving in the old city. Unless you are experienced in that sort of place, it can scare the crap out of you, with the tight spaces and the double-triple-quadruple parking.
I've not been to Brach/Bol.