Chimborazo volcano is the closest one to Riobamba. West of Riobamba is El Altar, an inactive volcano, and Tungurahua volcano. Going south there is the Sangay volcano, the least accessible and least studied volcano in Ecuador.
If you are willing to travel a little north from Riobamba, there is the Quilotoa caldera and Cotopaxi volcano. Both of them, are great tourist destinations.