Seems like your whole social life was dependent on your partner? Which tends to happen with men and is - in my and many others eyes- not healthy for a relationship. Invest in your social life, as you would in other aspects (work/health/fin), and start building.
The start up cost is vulnerability: if you meet someone you like to be friends with you need to essentially tell them 'I like you, I want to have you as my friend, I want to spend time with you, lets' hang'. The vulnerability and awkwardness are the initial price.
My favorites are the Clockarium, Museum of the Art Deco Ceramic Clock. (https://www.clockarium.org/ Check out the video, its is magnificent.)
And The Sewer Museum: Experience an authentic sewer, stroll along the Senne and discover the little-known but ever so important profession of a sewage worker.
Descend deep into the bowels of the city for this unique experience!
https://sewermuseum.brussels/
The start up cost is vulnerability: if you meet someone you like to be friends with you need to essentially tell them 'I like you, I want to have you as my friend, I want to spend time with you, lets' hang'. The vulnerability and awkwardness are the initial price.