1/ Reposition your Dock with just a click — The Dock tab in System Preferences lets you reposition your Dock on the left or right instead of the default bottom position. This is actually pretty useful with the wide format screens modern Macs have – you end up with more usable screen real estate.
But there’s a really fast way to do it too. You may realise the little divider line between the apps section and the folders section by the trash is draggable for an instant Dock resize (well, you do now) but if you hold down the Control key on your keyboard and then click (or just right-click it, if you have right click enabled in System Preferences>Mouse/Trackpad) that divider, a contextual menu appears (left) that lets you instantly relocate it. Note that if you have additional monitors to the left or right of your primary display, selecting the “left” or “right” positions will move the Dock all the way to the left- or right-most edge of your last display on that side.
2/ Bluetooth options — By default, the Bluetooth menu provides access to a list of currently connected and paired devices, the ability to disconnect a particular device and shortcuts to Bluetooth file sharing, Preferences, and the ability to disable Bluetooth on your Mac. But should you press and hold the Option key (usually labeled Alt on Windows and universal keyboard designs), and a whole bunch of new information and capabilities becomes available to you: the software and firmware versions of your Bluetooth chipset and devices, the physical address of your Mac’s Bluetooth interface in addition to the physical addresses of each paired device, signal strength information in the form of an RSSI (Received Signal Strength Indicator) value and, for Bluetooth speakers or headsets, the currently utilised multimedia codec. You can even create a diagnostic report for your tech person.
3/ Audio inputs — There’s a little volume slider at top right of your monitor. Slide it up and down to … change the volume, naturally, which you can also do with keys, typically the F11 and F12 keys. But hold down the Option key on your keyboard and click that volume icon and you get a list of available audio inputs to change quick-time. I use this many times a day, myself.
4/ Enable ‘Save As’ in Yosemite — It used to be easy to save a document with a new name in Mac OS X, as the Save As command appeared automatically beneath the File option in appropriate apps like Pages, Numbers, Keynote, and TextEdit. But for some reason, Apple hid it starting with Mac OS X Lion. However, you can make it reappear – click on the File menu in the upper left corner of your Mac’s screen, then hold down the Option key on your keyboard and the Duplicate File option magically turns into Save As. Now you can enter a new name for the document, choose which Folder to save it to, and even add tags to it if you wish.
5/ Use colours in iTunes playlists — iTunes 12 allows you to turn on a coloured background for open albums, movies and playlists. Unfortunately, the colour chosen isn’t up to you, but you can turn this on under iTunes’ Preferences under the general tab: tick ‘Use custom colors [sic] for open albums, movies etc’.
iTunes chooses the background colour based on some key colour of the item’s art. Sometimes it can look really nice, sometimes not so much.
This colour generally carries over to synced playlists on an iOS device, but may be altered, based on colour choices in Settings.