When I travel solo I always find the evenings more lonely than the days. During daytime you can always find something to do, even if it’s just walking around in the city. But during my trips I’ve figured out good things to do:
Have a cozy night in your hotel/hostel room. I usually to this the day I arrive, get myself some local snacks and maybe a tiny bottle of wine then I lie in my bed reading great books (additional tips: get cheap bocket books at a thrift store, that way you can leave them at you destination when you go back, in my experience there’s often a book shelf where you can leave your books in the common area).
Join a pub crawl, at least if you’re in a bigger city, not only is it a fun way to meet new people it can also feel safer to go with a group.
Late dinner – when I’m abroad I like to have a later dinner, order in dessert and a milkshake, and just sit and watch the people on the street.
I spent a couple of evenings at the Bella Italia restaurant near Piccadilly Circus last time I was in London.
Watch a show – I prefer going to a comedy show or a stand-up performance when I’m alone someplace new. Tickets are probably cheaper than, let’s say, musicals and it’s a more social thing.
Spa night – when I’m at home I never “have the time” to have a real spa night therefore it’s perfect to use some vacation time. In general you can pick up some mini-packages of hair treatment, facial masks and nail polish (and chocolate) at either the supermarket or the drug store. Ta da! Your own little spa at the hotel.