Bangkok already has loads of hop on hop off buses.
You wait for a bus, it comes and you hop on. You pay a few baht and then when it gets further along the route near your destination you hop off. If you want to go back you get the same bus going the other way, hop on, pay a few baht again and get off where you want.
Bangkok buses are super cheap, many are air-conditioned, some have location tracking so you can use the app to see when the next one will arrive etc just install the ViaBus app on your phone. It shows stops, routes, Bus GPS status and more.