I think I'd try and get really good at something - be it my job, or a hobby or an academic subject. I'd like to really give something my all.
I'd set personal challenges (as I do now but with different expectations) so would really love to walk the coastline of the UK, or do the C2C.
I would read huge amounts. Visit libraries, bookshops and get really really into fiction.
It's hard to properly walk in someone else's shoes. But much as I'm happily married with 3 children, I do fantasise about the complete opposite and how much energy I would have for other things.
I'm quite happy eating alone and generally enjoy my own company but can see that friendships and family relationships may become even more important and events like Christmas and birthdays may need more planning to ensure you have other people around.