All of the synesthesia I've experienced was caused by drugs. For the most part, it has been between visual and auditory, but other times it has been tactile and visual, or tactile and auditory. Never have experienced it with tastes or smells.
I've always have loved it when I experienced it. I would love to experience it while not in an altered state of mind.
I have HPPD (which is going away) - Hallucinogen Persisting Perception Disorder. For me this causes very easily forming afterimages and tracers - if I look at something, even for a tiny tiny tiny amount of time, I get an "afterimage" of the thing I was looking at. So it happens with pretty much everything I look at. When it was at its worst, it was very difficult to read text on a screen, because it would make afterimages off of the text and I would have all these weird colors forming in the middle of the text, often yellows. They were very distracting, but I did enjoy having constant visuals. I also had a moving translucent geometric pattern over everything I saw; in bright light conditions it was not very noticeable at all. In dark though, it became very apparent, even making it difficult to see in low light, at its worst. It was also multi-colored and changed colors. The geometric pattern now is really only there in the dark, and it isn't as powerful as it used to be.
I also have strange things going on with color perception, i.e. colors will morph into other colors if there are too many colors on a screen, like the blue and green here on WP keep reversing for me, i.e. the blue will turn into that same shade of green, and the green will turn into that same shade of blue again, and they will go back to normal, usually the moment I realize the colors are off. Interesting visual textures often have strange visual "glitches" occurring to them, with me. For example, with high contrast lines very close to each other, I usually see reds and blues between the lines.
Other people have things like seeing faces everywhere and in everything, like a hardwood floor or a wall. It is clear that they aren't real faces, but they just get recognized in places they normally wouldn't, sort of like how you recognize a smiley face icon
as a face, except recognizing faces everywhere.
I don't mind it though. It can be a little annoying at times but it's pleasant to me most of the time.