Voluntary work - great for the CV, can be sociable, good for confidence, many organisations pay for expenses like travel or DBS certificate. I have secured job interviews based on the skills and experience from being a volunteer.
Free online courses - lots of Universities and organisations offer skills and general courses. Again, great for the CV, mentally stimulating.
Fitness - don't overlook how physical fitness is good for mental heath. Lots of free online classes in all disciplines and fitness apps like Couch to 10k/half marathon, yoga, cardio etc.
Also good for the CV if you can show a commitment to sport and fitness. "I am training for a half marathon to raise funds for X charity in x month" etc
Charity Fundraising - also good for CV. There are lots of organisations that pay the joining fee for events so long as you can raise £x amount from your friends and family.
Also if you book an endurance event like running, walking, Tough Mudder etc, it gives you the motivation to train regularly for it.
For example, later this year, I am walking 23 miles for the Kiltwalk which will take me around 10 hours so I must train properly for this. The charity I am raising funds for has paid my entry fee and will give me a tshirt.
These will give you transferable skills for employment so depending on what you do - team work, planning, self development, leadership, customer service, communication etc