I mean it depends on where you live. But sports clubs are easy enough to find if you just search for your sport and area.

Same I would guess with other hobbies.

Fitness groups no idea to be honest.

Volunteering: there are usually quite a few forums and such discussing volunteering opportunities. In my country the biggest ones are the volunteer fire department, volunteer first aid (red cross, Johanniter, ASB, Malteser, etc.), technical relief (THW), and different organisations regarding the homeless and poor (biggest ones probably are the Bahnhofsmission and Tafeln) - this is all Germany specific but I'm sure there are somewhat similar things in other countries, too. For smaller things the are often even websites from the local government where you can search for volunteering opportunities interesting you, by topic.