At a minimum, an existing building should allow entry to their facility.  Some readily achievable barrier removal examples at entrances could be:

The old hardware at the door could be converted to a lever for easy opening.

If there are steps or higher thresholds at entrances, adding a ramp will make the entrance accessible. 

The next thing that we should provide is an accessible restroom.  This may not be readily achievable to fix the entire restroom, but there are elements that can be fixed easily: 

Adding permanently fixed objects below large paper towel dispensers which could be  protruding objects along the walls.

Wrapping exposed pipes at lavatories will assist people in wheelchairs and avoid injury due to sharp or hot pipes

Some barriers may be more expensive, such as either fixing existing non-compliant curb ramps or adding new curb ramps along accessible routes;