The Simon Community help some of Glasgow's most vulnerable: https://www.simonscotland.org/
If you are at the uni, then GUSH offer volunteering opportunities too:
https://m.facebook.com/pages/category/Nonprofit-Organization/Glasgow-University-Service-to-the-Homeless-GUSH-1902919743302843/?locale2=en_GB
And something completely different:
https://treesforlife.org.uk/