sw628, I had regular contact with my T outside of therapy and his office for about 2 years when he served in another professional role for me. We met in a few different locations, usually with others. Sometimes it was informal, like sitting together in the lobby of an office building. Sometimes it was more formal, like being in a conference room with others.
Before we began this second role, I did have a worry that I would not know how "to be" with my T outside of therapy. We only had interacted in his office, in that very special way you can interact with a therapist--revealing a lot, expressing feelings, etc. In those days, I was crying a lot in therapy. It seemed like I would be fine all week, get to therapy, and then suddenly find myself sad. It was safe to cry with T. I had this worry that on the outside, I would be all emotional with him too, in a situation that I did not want to be emotional in. I worried I would be trained like a dog--if I was near T, I would be emotional and cry. I didn't know if I could be "normal" with him (more reserved and contained).
What helped me decide if this was going to work was that one day in therapy, he asked me to come outside with him to his car to get something. (It was either that, or leave me alone in his office for a while.) I jumped at the chance, because I immediately recognized it as a chance to be with T outside of his office and see how it felt and if I could interact normally with him. So we went outside, we walked to his car, we got in it, and we drove back. I discovered I could "be" just fine with him outside of the therapy context. I had no trouble with it. I didn't just look at him walking next to me on the sidewalk and burst into tears, LOL. Based on that experience, I decided to go ahead with the second role. And it all turned out just fine. I was very comfortable with him outside, and I didn't get all emotional or cry inappropriately just because he was there. But when we did therapy, I could still be vulnerable. So, it all worked out fine. We are back to doing just therapy now, so I don't see him outside anymore, which is fine. It all worked out very well and did not negatively affect our therapy relationship.
I think the birthday dinner with your T sounds really nice. It sounds like it will be just fine.