While it’s not obvious, you can use the MiniPlayer in iTunes 11 when you’re in full-screen mode. To do so, switch out of full-screen mode, set iTunes to show on all desktops (right- or Control-click on the Dock icon, select Options > All Desktops), open the MiniPlayer as a separate window (Window > MiniPlayer), then click the full-screen button on the main window.
Note that if you have the MiniPlayer set to float above other windows (in the iTunes Advanced preferences), then it won’t display when you switch to iTunes. This makes sense, of course, because you don’t need it then. Also, this is the only way to get the MiniPlayer to work across spaces, if you use them. If iTunes is not in full-screen mode, then the MiniPlayer only displays in the same workspace as iTunes.
The second tip is about two ways you can quickly add songs to the Up Next list.
First, just drag an item from the iTunes library onto the iTunes LCD (the display at the top of the window that shows what’s playing). This can be a single song, an album, or a playlist. The iTunes LCD will show a blue border when you bring the item over it, and the Up Next icon will flash with art of the item you have added.
The second way is to press the Option key and hover your cursor over an item. The track number next to its name will change to a plus-sign (+) icon. Click that icon to add that track to Up Next.