Span is a player with whom the Twins have a win-win situation. If you keep him, he's got a good contract and he fills several needs (speed, leadoff, CF defense). If you trade him, he should bring back good pieces. It was noted in a few other threads what similar trades have yielded recently (Michael Bourn, Nate McLouth, Hunter Pence). The Twins should try to get a deal that beats those ( though the Pence deal probably will be hard to match). If they can't get an offer better than those, keeping Span is still good for the team.
Doumit is a tough one, because he fills a lot of holes for the Twins as a switch hitter with power. His defense is not so great, and he's only on a one-year deal. If the Twins don't trade him, I bet they will give him another contract (or at least try to re-sign him).
With Liriano and Pavano, is kind of the opposite of Span. They will not be cheap to re-sign when their contracts expire, and they are probably not worth much on the trade market right now. The Twins aren't likely to get much for either one, but they are probably worse off if they keep them (and probably would have been better served trying to trade them sooner, though that's really only in hindsight).