Not rails specific, but I dislike the idea of storing too much stuff in a cookie. Instead I store it on a database on my web site, and just put a primary key value in the cookie. That way you can store as much or as little as you like without transmitting too much over the net. The disadvantage of the "no registrations" approach is that they lose all their stored data if they change to a new computer, or even a new browser on the same computer.
What I do in my app is to store a cookie with a "session id". Then I have several tables, one stores one-off data like the last date that session id was seen, and others store multiple items per session id. I have a "session_states(session_id, state)" table, for instance, that stores one record for each state a person chooses from a list of US states.
One reason for storing the last date is that I purge any session ids from the database that haven't been seen in two years (because I give an expiration of +2 years when I create the cookie).