The subject disclosure is directed towards a web service that maintains a set of models used to generate plans, such as vacation plans, in which the set of models includes models that are authored by crowd contributors via the service. The models include rules, constraints and/or equations, and may be text based and declarative such that any author can edit an existing model or combination of existing models into a new model. Users can access the models to generate a plan according to user parameters, view a presentation of that plan, and interact to provide new parameters to the model and/or with objects in the plan to modify the plan and view a presentation of the modified plan.