In the US, kissing on the cheeks is not common unless you are in show business or with relatives or close friends. That's my impression. So I think doing that potentially crosses more boundaries.

Hugs are common in therapy and outside of it, so I don't think they are considered to be crossing a boundary.

Holding hands, as others have said, seems either too intimate or child-like. I'm guessing most therapists would think it crosses boundaries.

However, I'm generalizing. Every T is different. Every client is different. Boundaries depend on what feels comfortable on both sides. My T has made holding her hand an important part of my therapy so it obviously doesn't cross her boundaries.