Here is how I would rank in order, highest to lowest, the trade value of the Twins positional players:
Joe Mauer
Justin Morneau
Denard Span
Jason Kubel
Michael Cuddyer
Carlos Gomez
Delmon Young
Nick Punto
Alexi Casilla
Brendan Harris
Matt Tolbert
Brian Buscher
Looking at that list makes one realize that if the Twins make a trade this offseason, it likely won't be a position player. Mauer and Morneau are off limits, and after Delmon Young, you wouldn't get much in return. Brian Buscher, Alexi Casilla, etc. aren't going to bring back anything great.
The one option they have is to trade someone from the outfield of Span/Kubel/Cuddyer/Gomez/Young. I could see an argument for trading any one of them except Span. Most of the Twins top positional prospects are outfielders, and they will have one at AAA this season in Rene Tosoni. I'd argue that Tosoni could step in and provide as much value overall as Young and Gomez did this season.
I wouldn't trade Young or Gomez at this point, unless they got a lot in return. I just don't think their value is high right now, so they wouldn't get much for them, and trading one of them could come back to haunt you a la Bartlett/Garza.
Kubel and Cuddyer help form part of the heart of the order, but I would trade one of them for a player of similar value at 3B or SS or 2B or a #2 type starting pitcher. I think with the glut of outfielders coming up through the system, the Twins are going to have to trade some outfielders at some point in the upcoming years to make up for other deficiencies that they have.