I did Grenoble for 5 days last year. In between we spent two nights in Annecy on Lake Annecy. We used public transport to get around. We thought that 3 nights in Grenoble was too long. It is a lovely town but little to do except potter about. Annecy was lovely but the weather was a bit grim (May) but there are boat trips out on the lake. We didn't see much for children in either place.