We consider a class of models for dynamical dark energy: we construct a general effective dynamics for diffeomorphisms of spacetime, in a fixed external metric. Though related to familiar models of scalar fields as coordinates, our models have subtly different properties, both at kinematical and dynamical level. The energy-momentum tensor consists of two independently conserved parts. The background solution is the identity diffeomorphism and the energy-momentum tensor of this solution gives rise to an effective cosmological constant. We will also discuss possible applications to the construction of relational observables. Based on arXiv:2012.04507.