Receiver Jordy Nelson started the day as a free agent. He continues to be a free agent.
His ceremonial retirement with the Packers did not include signing a meaningless one-day contract because, frankly, signing a one-day contract does have some meaning - more meaning actually than one share of Packers stock. (Cue the Packers shareholders who resent having the meaninglessness of their framed piece of memorabilia pointed out.)
By signing an actual one-day contract and retiring, a player lands on the reserve/retired list. The team holds his rights, and can control his rights. So if a team - like, say, the Bears, Lions, or Vikings - would need his services at some point this year, the Packers would have to release Nelson's rights in order to allow the move to happen.
Even if Nelson currently has no interest in playing, there's no reason to tie himself to the Packers because circumstances could change at some point between now and January. A truly retired player (like Patriots tight end Rob Gronkowski) has a Week 13 deadline for emerging from the reserve/retired list. Nelson, like any free agent, can sign with any team at any time, up to and including the week of the Super Bowl.
And even if the Packers would gladly release Nelson's rights if/when he wants to play, he'd be required to pass through waivers if he's released after the trade deadline. In 2002, the Chargers prevented Deion Sanders from climbing aboard the Raiders' Super Bowl bandwagon by making a waivers claim when Washington released Deion from the reserve/retired list after the trade deadline.
So Nelson can ceremonially retire with the Packers and easily unretire whenever he wants, if ever he wants. For anyone who has yet to completely and totally slam the door on playing, that's the way to do it.