Access to wheelchair skills training is important for the mobility and independence of wheelchair users, but training rates are low - particularly among young people. In this paper, we present \textit{Geometry Wheels}, a movement-based experience prototype to explore the potential of interactive technology to support basic wheelchair skills training for manual wheelchair users, designed with the support of occupational therapists. Results of an evaluation with 15 participants (10 young wheelchair users and 5 parents) show that interactive systems can deliver engaging and challenging activities that encourage wheelchair navigation and activity. However, the project also revealed challenges in designing for individual differences in physical abilities, in conflicts between children's and parents' perceptions of ability, and barriers to home use. We outline strategies for the design of rehabilitative technology to help young people with disabilities build physical abilities
