So I don't think there are necessarily "better" places for solo travelers.
When I retired, I broke out of my England/France/Netherlands bubble. I've traveled solo to places as disparate as Singapore and Switzerland, Canada and Cambodia, Australia and Austria (no kangaroos in Austria :-). I went mainly to cities because they have so much to offer (history, art, architecture) and they are easy to get to by plane and train. About 5 years ago, I looked into renting a car in Europe and found it wasn't too expensive for my budget (I try to keep costs down so I can travel more) since I am comfortable driving a manual shift. A few days with a car lets me explore small towns and villages, take scenic drives in Spain, France and Italy, and access more remote sights - like the buffalo farm in Campagnia. So I don't think there are necessarily "better" places for solo travelers.