You might want to be a volunteer.
There is a difference between unpaid employment (voluntary work) and volunteering, and you should always check with the organisation which offers you a volunteering opportunity whether it can be regarded as unpaid employment. This is because time you spend doing unpaid employment counts towards your maximum number of hours of work a week.
Students with a work prohibition must be particularly careful not to undertake work that could be regarded as unpaid employment. This includes Tier 4 (General) students sponsored by a private institution, an independent school or by a publicly-funded further education college and short-term students. Taking unpaid employment would be a breach of your work prohibition and is treated very seriously by the Home Office - your immigration permission will be curtailed (cut short), you could be removed from the UK and you will have difficulties returning. If in doubt, do not undertake the work.
The Tier 4 policy guidance confirms that Tier 4 students can volunteer and explains how the
Home Office differentiates between 'voluntary work' and 'volunteering'. For example volunteers do not have a contract and are not paid, though reasonable travel and living costs can be reimbursed. Volunteers usually help a charity, voluntary organisation or public sector organisation.
Although Tier 4 students may not work as an entertainer or as a professional sportsperson (including sports coach), it can be possible to undertake such activities as a volunteer. You must be doing it as an 'amateur' if it is not an assessed part of your degree course - see Professional sportsperson, Entertainer and Studying music or dance at degree-level.
There are many ways of finding volunteering opportunities, and your Tier 4 sponsor can probably help. Here are some links to get you started:
National Council for Voluntary Organisations (England)
Volunteer Now (Northern Ireland)