Run bikes without pedals are the best solution, I think. They can develop sense of balance and later change to a bicycle with pedals.
When I was about 4 my elder brothers thought it was about time for me to learn riding a bike. So they unscrewed the stabilisers, lifted me on the saddle and pushed me ahead. All day long. Main difficulty was that the alley they had chosen for that was a dead end and i had to learn a u-turn on the very narrow alley or go down some stairs by bike. In the afternoon I could ride down stairs to some degree and was pretty proud of that. But my mother was shocked the way I looked in the evening. I wouldn't recommend this approach all in all.