Residence card address while travelling?

Hello!

I moved to Japan 2 weeks ago on a 1yr working holiday visa and am currently staying in a sharehouse in Tokyo, so I put this address on my residence card. But in 2 weeks time I will be travelling around Japan for a few months, staying at each place for 1-3 weeks. And I haven't decided where I'll go after that.

So what should I do about the address on my residence card? Can I leave it as-is until I get a more permanent address? Or, could I put down the address of my friend living in Tokyo?

Bonus question : I will be flying back to the UK for Christmas. Am I right in thinking that I don't need to fill out a re-entry permit under the new system?

Any advice would be great!!