I would be a bit cautious about the WWOOF program. Not that the program is illegal or bad, but my impression is that rules/regulations have changed quite a bit in recent years, and WWOOF hasn't always had the most up to date information. Nor do all the hosts who offer WWOOF placements necessarily know the current regulations and/or offer opportunities that are necessarily what people expect. Just because WWOOOF says it's legal doesn't mean it's legal.
Seems to me I've read some stories in recent years about people who signed up for WWOOF programs that turned out to not to be legal. (This is an old CIC document which was issued because of issues with WWOOF: http://www.cic.gc.ca/english/resources/manuals/bulletins/2013/ob503.asp) Not something you want to find out once you've arrived. I would do your research carefully and if you have questions, contact CIC.
That said, I wouldn't get to caught up in CIC's definitions.
You need to be honest - you will be asked the purpose of your trip and if you are spending two weeks at a farm, say that. If you say you are just a tourist, then the farm work comes up later, it's going to be a red flag. Explain that you are volunteering and not being paid other than room/food. Do some research on the farm so you can provide a brief description - i.e. where it is, who owns it, what is grown/raised on the farm, how big it is (i.e. acres) and what you will be doing. The owners should easily be able to provide that info - if they won't or are cagey, it's probably a sign that something is not legit. It would be a very good idea to have a print out of an e-mail from the owners laying out the dates and what you will be expected to do and that it is a volunteering.
From the CIC website:
"if a tourist wishes to stay on a family farm and work part time just for room and board for a short period (i.e., one to four weeks), this person would not be considered a worker. Work on a farm that is expected to extend beyond four weeks would require a work permit."