How to manually play a note block's sound in creative without breaking it?

Solution 1:

Power it with redstone. Most commonly, people play songs by having a lot of note blocks linked together with repeaters to create controlled delays, creating the rhythm. There are a large number of these videos on YouTube that you can also see examples of this with.

For a very short example of this, put a repeater down that points into your note block. Then, power that repeater with a torch. Ta da! Sound.

Solution 2:

Powering it by redstone is the way. Now how to power it with redstone in a way that is quick, easy and allows playing 'manually' in Creative? Noteblocks are great as player inputs, completely regardless of producing sound - right-clicked they produce a state change that can be noticed by an observer and provide the redstone signal that way.

This would be a simple "piano" you can use in creative - the right-side blocks are muted by placing buttons on top of them (any other block will do, buttons are just pretty unobtrusive). You activate them by right-clicking. The rear blocks are actually giving out sound, and the blocks underneath change the instrument. The solid blocks between the observer and the noteblock are so that the noteblocks in the back are soft-powered (activated) so that only one block makes sound on input; without these the hard-powered block would activate two adjacent blocks as well.