Canada tourist visa online appication

I assume you mean question 4, where it asks where you're visiting. As per the link, you should:

Type the following information about the person(s) or institution(s), including schools you intend to visit during your stay in Canada:

• Name (includes the name of a person or a name of an institution)
• Relationship you have with them (friend, family, co-worker, etc.)
• Their address in Canada (street number, street name, city or town and postal code).

Remember, at this point they're trying to see what you've planned, where you intend to go and so on. If you're visiting a friend, put their details. If you're just being a tourist, put some of the destinations, maybe the accommodation for the first night that you've booked (even if you don't book the other nights yet, they like you having that one done, especially on arrival).

This does NOT set it in stone. If you change your plans, that's ok. It gives them some evidence that you've thought about your trip and have an itinerary. It also means at the border, they can ask what funds you have to support you, and if say, you have a small amount, but are planning on staying at top class hotels and going on amazing tours, it might raise questions.

It also means that in the event they need to contact you during your trip, this might help them do so. They can also contact your friends/contacts to confirm that what you've said is true - so make sure your friends know you've written their name down...