PI has always been open during the day. There are several shops on the island that are open all day and can be fun to browse.
In the old days, the entire island was closed off after 7pm when the clubs opened, and you could only get onto it if you had a PI admission pass.
About 2 years ago, a new policy called "Pleasure Island Ungated" was put into place, meaning that anyone can get onto the island after 7pm, but you still need the pass to get into the clubs. Once you get a pass, you get a wristband that shows that you have the pass.
Pleasure Island passes can be bought at the PI gate. If you have a MYW pass with the Water Parks and More option, you can use a Plus feature to get a PI pass.
The rest of Downtown Disney is simply a Disney-themed shopping plaza. There is no admission and no charge to park (unless you valet). DTD has lots of stores, restaurants, and eateries, and the AMC Pleasure Island movie cineplex (24 screens, just like your local shopping mall movie theater), and the Cirque de Soliel: La Nouba show (which is a ticketed show that you cannon use any form of park pass to get into).
To get a better perspective on DTD and it's three sections (the Marketplace, Pleasure Island, and the West Side), click the link in my signature to get to my maps page and have a look over the DTD map.