When iTunes 11 first came out, the illustrious Bryan Chaffin pointed out that it's easy to restore the traditional sidebar by choosing View> Show Sidebar. We all breathed a sigh of relief and put down our pitchforks at that point, didn't we?
The formerly omnipresent iTunes sidebar.
As time goes on, though, I'm finding that I'm using the sidebar less and keeping it hidden more, so if you are, too, here's how you can add music to playlists without having it on. It's pretty darned easy, actually—all you've gotta do is pick up a song, album, or artist from your library when the sidebar isn't showing, and as soon as you do that, iTunes will obediently slide open a drawer on the right-hand side of your window.
Within that drawer are your playlists (and any attached devices!), available for your dragging-and-dropping pleasure.
After you drop your item onto the playlist of your choice, the drawer disappears, leaving your iTunes window as clean as it was before.
If you prefer, you can also add something to a playlist by selecting the arrow that appears when you hover over any song, album, or artist. Then you'll click on Add To to see your playlists and put your selected item within one:
Hey, it's like I don't even need the sidebar anymore. Wait, not really! Please don't take away the sidebar view, Apple! I'll be good.