2017 Wheelchairtravel.org blog post: 13 BEST WHEELCHAIR ACCESSIBLE HOP-ON HOP-OFF BUS TOURS The two leaders in this arena are Big Bus Tours and City-Sightseeing, with accessible buses serving wheelchair travelers in more than 50 cities across 6 continents. While not every city has a 100% accessible fleet, most buses do have a wheelchair accessible ramp and dedicated space onboard for parking the chair. Article also mentions places that are favourites to visit along the route. Berlin, Germany ; Cape Town, South Africa ; Chicago, USA; Copenhagen, Denmark; Dubai, United Arab Emirates; Hong Kong, China ; Kuala Lumpur, Malaysia; Las Vegas, USA; Madrid, Spain; Moscow, Russia ; New York City, USA ; Stockholm, Sweden; Washington, D.C., USA
https://wheelchairtravel.org/13-best-wheelchair-accessible-hop-on-hop-off-bus-tours/