Unless he has a storage locker somewhere full of things, Dean doesn't have a lot of possessions. The ones he carries with him tend to be things that were previously owned by someone close to him, now gone. Most of these items were John's, arguably the person he most looked up to and wanted to emulate. He has his father's journal, and for a while he had his father's leather jacket that he claimed in the Pilot. He's driving John's car, although a case could be made that she was always Dean's and John just kept her for a while. The journal is pretty much a representation of John's life as a hunter and the jacket says a lot about who he was in other ways.
Dean's had a few other possessions that seem to be constants. They represent huge pieces of his life. One is his gun that he lives by. Another was the amulet Dean wore given to him by Sam. It pretty much represented his relationship with Sam until it got tangled up with Cas's search for God. He's had a few other things that must have had some meaning to him although we never knew their stories. His silver ring from the first several season must have had a heck of a back story (and I figure it's somehow become a permanent part of the Impala, by chance or design).
At the beginning of this season, Dean retrieved Cas's trenchcoat after he disappeared into leviathan goo. The trenchcoat was as closely tied to Cas's identity as his wings. We saw in a later episode that it was in the trunk going along on Dean's journey.
Looking through caps from the last two episodes, it appears that Dean is now carrying Bobby's flask. He pulled it out of Bobby's duffle when Sam answered Bobby's phone at the beginning of Adventures in Babysitting.
In Time After Time, when he is arrested, it is in a pile of his belongings on the table in the interview room. Later, when he gets a new suit, he puts his items in his new pockets and he holds the flask for a second.
I don't really think that the flask represents Bobby in the same way as Cas's trenchcoat symbolized him. It's a pretty strong symbol nonetheless. Just by the nature of it's use, a flask is a pretty personal item and I'm wondering what this flask means to Dean. He and Bobby spent quite a bit of time in the last couple seasons drinking together as they worked through some rough situations. Alcohol has been taking a more prominent place in Dean's life as losses have accumulated and the stakes have gone up. He didn't need a new flask as he already had one. Alcohol played a pretty big roll in Bobby's life. Sheriff Mills initially called him the town drunk. He even produced his own. I hate to see Dean end up heading down the same path but at this point it certainly looks probable.