  Macroscopic mechanical qubits are fundamental both to test the
classical-quantum boundary and present suitable candidates for quantum
information processing. Motivated by these, we propose a feasible probabilistic
scheme to generate an on-demand mechanical qubit, as well as Schr\"{o}dinger's
cat and Fock number states. In order to accomplish this proposal, we study an
open dispersive spin-mechanical system in the absence of any external driving.
The procedure is solely based on spin post-selection in the weak coupling
regime. Through this scheme we demonstrate that the achieved superposition is
closely related to the amplification of the mean values of the mechanical
quadratures as they are associated to the maximum quantum coherence.
