Tattoo for Dad

I will be getting a tattoo for Dad soon. I was originally thinking I'd get it on the back of my right shoulder, to mirror the one I got for Mum, but now I'm thinking I might put it with Mum's tattoo, and then have one each for my two brothers - but that means I have to figure out what would be best for them, and how it would all go together.

Mum's is a *mostly* realistic-looking moon (as in, not a hard line icon style like my other two, but with shading and some colour). I had planned a sailor's knot for Dad (which would have to be somewhat realistic to go with the moon, rather than a simple sketch, right?). I think the younger of the two brothers would pretty much have to be represented by a guitar, I just don't know what would be right for the other - ideas?

Keep in mind I'd have to actually want it permanently on my skin.