In order to be able to evaluate beforehand the impact of a strategical or tactical move, decision makers need reliable previsional systems. What-if analysis satisfies this need by enabling users to simulate and inspect the behavior of a complex system under some given hypotheses. A crucial issue in the design of what-if applications in the context of business intelligence is to find an adequate formalism to conceptually express the underlying simulation model. In this experience paper we report on how this can be accomplished by extending UML 2 with a set of stereotypes. Our proposal is centered on the use of activity diagrams enriched with object flows, aimed at expressing functional, dynamic, and static aspects in an integrated fashion. The paper is completed by examples taken from a real case study in the commercial area.