Replying to Knee strengthening

Does anyone know some Knee strengthening exercises that a young person with messed up knees could do in preparation for the upcoming ski season. Last ski season my knees somehow got messed up and i wasn't able to ski for a bit but i went to the physio and did some strengthening and they seemed to sort themselfs out, this physio is away at the moment so i was just wondering if anyone has any recommendations for knee strengthening?
Kind regards from Sir EPfanboy of the EPfanboy manner.