I've had quite a few holidays on my own. I went to Dublin, for example, and it's a great city for getting around. There are loads of things to see (if you like museums). I also did a three week road trip around the US once.
I have to admit that I rarely spoke to anyone except waiters, bartenders, hotel receptionists... but there was no point pretending to be gregarious when I'm not.
If I was going on a solo holiday now, I'd be tempted to do an activity holiday... quite fancy wine tasting or bread baking. I'd even consider a buddhist retreat.
I have tried one activity holiday... children's book illustration (which I was into at the time). It was led by Michael Foreman and it was a great week.
The main thing, if anyone asks "Are you going on a holiday on your own" with a pitying look in their eyes, say "You're damned right I am."
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Steve J
Unkind tongue, right ill hast thou me rendered
For such desert to do me wreak and shame