How often do you make stops where there would be restroom access?
Well, our group had a person who needs the restroom frequently, and she never had any problems in that area. More than enough places and times to get to a bathroom. However, one stop you needed some coins in order to access the restroom.
There was a stop about an hour after starting, in a small town/rest stop area. Then say an hour to a waterfall, without facilities, then another hour to the geyser area with food, shops and facilities. Then another 1.5 hours say till a stop in the national park, with full facilities. Then say 1.5 hours back to Reykjavik.
Get an answer from Golden Circle Full Day Tour from Reykjavik by Minibus staff and past guests.