Some things which are free / cheap -
Running / jogging / walking - you need a pair of trainers yes but if you can’t afford brand new you will absolutely be able to pick up a pair on eBay or Facebook martlet place or Vinted etc. I sold a brand new pair of £80 trainers on vinted for £10 - they’d only been worn once by DP. So there are cheap bargains to be had. Then download the couch to 5k free app or look up some beautiful country walks and get going.
Alternatively there are lots of gyms which charge between 10-20£ a month.
Reading groups / book clubs (online or in person) - borrow the book from library or get into a book share group.
Podcasts and YouTube content - mostly free or sometimes a low subscription charge for things like Spotify. There is content for any interest you may have - clothes, music, cooking, crafting, fitness, comedy - literally anything.
Choose a podcast of a subject you enjoy, get your shoes on and get outside in the fresh air.
Upcycling - nominal costs for second hand furniture from junk shops and some paints / materials.
Cold water swimming - if you live near a lake or the sea you might find there is a local group of cold water swimmers. I’ve heard amazing things about it.
Rambling / hiking - similar to running, it’s free other than some suitable footwear, there will likely be local groups if you don’t want to do it alone.
Volunteering - put your time and effort to a good cause and volunteer a few hours a week.
Night classes or online learning - usually there is a fee but for example my local university does summer classes for £50 for 6 week worth of lessons - for example creative writing or computer coding, there were lots of options.
Alternatively the website Udemy often sells their courses for 13.99 - I bought two courses which will both benefit my career for 10£ each. There are loads of subjects to choose from.
Mediation / yoga - endless free content on YouTube.
Ultimately OP it’s about really digging deep and asking yourself what would make you happy. Then putting the change in motion, step by step.