If dad is a musician, there are a few cool gifts you can pick up for him. For the aspiring video star, there are lots of webcams and USB microphones to choose from. A good camera and microphone can make a big difference in the production value of even the simplest video.
For the guitarist, you might want to look at gadgets like the iRig or the Peavey AmpKit. These let your dad plug his axe into iOS devices like the iPhone or iPad to make them sound like they're hooked up to an amp. He can even mimic effects pedals and tweak settings to get the tone he wants.
Another simple gift is an electronic tuner. You can find tuners for lots of different instruments. They detect the vibrations given off by a musical instrument and can let you know if you need to make any adjustments.
And just in case you want to preserve marital -- if not musical -- harmony, consider buying a nice pair of headphones so that dad's musical output doesn't filter through the rest of the home.