You can, on a Canadian passport, enter without an Standard Visitor Visa, as can anyone holding one of the non-visa national passports.
You can do up to 30 days volunteering, as long as the sponsor is on the Charity Register. This is not restricted to only non-visa nationals, individuals holding a Standard Visitor Visa can also benefit from the same rule.
It's explained in Immigration Rules Appendix V:visitor rules:
APPENDICES TO THE IMMIGRATION RULES FOR VISITORS
VISITORS APPENDIX 3. PERMITTED ACTIVITIES FOR ALL VISITORS (EXCEPT TRANSIT VISITORS)
- All visitors are permitted to undertake the activities listed in paragraphs 3 – 27 of this Appendix provided they meet the requirements at V 4.5- V 4.8. Visitors coming to the UK under the ADS agreement may only do activities in paragraph 3 of this appendix.
- Visitors may only receive payment where allowed by V 4.7.
Tourism and leisure
3. A visitor may visit friends and family and / or come to the UK for a holiday.
4. A visitor may undertake incidental volunteering (i.e. the main purpose of the visit is not to volunteer), provided it lasts no more than 30 days in total and is for a charity that is registered with either the Charity Commission for England and Wales; the Charity Commission for Northern Ireland; or the Office of the Scottish Charity Regulator.
Charity Commission for England and Wales
Charity Commission for Northern Ireland
Scottish Charity Regulator
NOTE: Volunteer work, paid or unpaid, of any sort is illegal unless the employer is certified by the Charities Commissioner. The UK has several farms where a person can 'volunteer' for room and board in return for work duties on the farm. Most of these farms are NOT registered charities and so they are useful for EEA nationals and other nationals on a Youth Mobility Visa. If you are not an EEA national or have a T5 (YMS) visa and get caught at one of these farms, you will be in trouble and the farm operator will be heavily fined.