If you are going to re-sign Glen Davis, the best contract to sign him to, assuming the rules haven't changed under the new CBA, is a one-year contract with a team option for a second year.
The option makes him easier to trade because a player who is on a one-year contract and will have Bird rights at the end of the contract will lose those Bird rights and thus is permitted to veto trades. However, if you add a team option, the Celtics can invoke the option before trading him.
Would Davis agree to such a contract? If BBD is the best option, would it be worth overpaying him slightly to get that additional roster flexibility? I believe that the ideal contract size to maximize roster flexibility is to give Davis just enough so that he can be packaged with a minimum salary veteran for a player previously signed to a full MLE contract.