If you get a few students together, you can help the environment by picking up rubbish from the beach/rivers or in your own neighbourhood. Land pollution has become a big problem and it affects wildlife - animals getting stuck in plastics etc
You can also visit centres for old people to talk to them as they tend to get lonely. Actually you'd do more listening than talking! They have a wealth of knowledge to share, their experience and their stories but just no one to share it with. You can play games with them (cards, dominoes, chess).
Helping kids from low Socio Economic Status is something else you can do. You can help them with learning or just be there to give them support as they come from dysfunctional families. Khairah has shared her experience of doing this, so have a read through when you have time.
To be honest, there are lots of things you can do without money. You just have to be ready to give away your time.