The first logical option would be the veteran that spent some time with the Chargers this preseason but ultimately was cut. Jones is similar in skill set to Keenan Allen. He has great hands and has the ability to make amazing catches. He also is a solid route runner that can be a chain mover like Allen was. Jones is a logical fit as he already knows the playbook and has a similar skill set as the receiver they are losing.
Washington is another intriguing option. He has more speed and is more of a deep threat. But he is also a veteran that should be able to learn the play book and can get separation to offer Rivers a solid, reliable pair of hands that knows how to get open.
Nicks has bounced around a little bit, but has a solid skill set when he is able to put it all together. He has good hands and is able to high point a ball to make big plays. He also is a crafty route runner that has a knack for getting open.
Obe of the best options that isn't already on the team, Colston is a perfect fit for the Chargers. He is a solid possession receiver that can be used in a role similar to Anquan Boldin as he is a chain mover that will make the catch. He is a solid compliment to Travis Benjamin that knows how to win and would also be good on the locker room.
Avant is a solid player that is a solid, veteran receiver that is familiar with similar pass concepts. He could come in and be a solid player. He does have a knack for heartbreaking drops though, which will hurt his signing ability.