I've been retired for about 25 years and in that time I did Books on Wheels - choosing and delivering books from the Library to housebound people; a foodbank, and currently and very enjoyably, a volunteer with The Reader, running a weekly reading aloud group - it's a free, pleasurable way of reading with other people. You make friends and know you're helping people - combatting loneliness for instance, at the same time. Additionally with The Reader, I'm about to begin reading one to one with a cared-for child, something which can make such a difference to children who've had a difficult start to life.
Like another poster, I think you have to think very carefully about what you are prepared to do, and how much time you are willing to give. Jumping in feet first and then having to stop giving your time because you find that it doesn't suit you or you don't have enough time to spare, leaves you with a guilty feeling of letting people down, and the point of volunteering is to help others as well as yourself.
Choose carefully and you will have a great sense of wellbeing. Good luck!