I've done a decent bit of travelling and felt great joy during those experiences, but the one moment that jumped to mind when reading this thread is from my teenage years.
My first relationship was online and after about a year of long distance we met in person. I flew over and stayed with her family for a few days. I can only describe the emotion I felt as pure euphoria, distilled and undiluted. I'd never tried any drugs but for those few days (and especially the first) it felt like I was both high and in a dream. I've tried to speak about this feeling of euphoria before but I think people always assume I'm talking about sex when that's not the case at all. It was a mixture of anxiety and elation and like a daydream had come to life.
The come down (if it can be called that?) was not quite as intense but hit hard too. For a couple days after returning home I kept questioning if it had all been a dream.
I've never had an experience like that since and I'm not sure I ever will.
I don't know if it counts as feeling "fully alive" but it's what came to mind.