Remember to level all of your great buildings before you go, so there isn't anyone hanging with fp sitting on them.
You might also trade higher goods to players who need them for building at a 2:1 ratio in their favor for the lowest goods they have before donating the remainder to the guild treasury.
You can donate all of your medals to the guild treasury.
You could spend any diamonds you have remaining on camps in GBG to help your guild too.
Do all the things Kranyar says AND sell all your non-premium buildings.
Put all your Happy Buildings from your inventory into your city.
Fill all your empty spaces with trees.
Leave some diamonds in your account, this will keep you registered should you change your mind.
Remove/release all your friends.
Post a message in your Profile.
If you have a method of payment on file, delete it.
Disconnect your remaining buildings from your Town Hall and go AFK.
Unless you are absolutely positive you are not coming back, I would open your city at least once a month.
If you come back to stay, you'll be able to easily resume.
If you never come back, your city will be removed by INNO eventually.