Riva is at the far end or the lake, and from there Arco is around 20mins by bicycle!
It was raining when I went for an afternoon so I wandered over to the massive covered outdoor wall, plenty of routes and boulder probs there, ask nicely and they will let you have a go.
As for proper bouldering, im sure there is loads, ask for a topo at the big outdoors shop in Arco.