They will likely ask what purpose of visit is. Saying tourism / visit is not a lie if that is one of purposes. Fro those with eg French passports, that is often the end of it, although may also ask where staying / with whom / visiting whom. Length of stay is another question; fine to say a couple of months.
Some here believe having a return ticket is a must - I think that's no longer the case as so common to get tickets on internet, or - esp for European visitors - say plan to go to states and return from there. (This may be less applicable for visitors from places where tickets are more difficult or very expensive).
Note, if travelling with spouse, questioning may get more extensive. It's also okay to say plan to apply for sponsorship 'at some point' but 'haven't finalized plans.' Obviously also helps if someplace and job to return to; again, can be vague about 'things to deal with / affairs to wrap up [at home] before moving'.
Important to note: don't lie. You just don't have to go into detail or volunteer that much extra. Brief factual answers.
If you're very worried, travel separately from spouse.
My personal experience and knowledge from contacts is that it is fairly rare* that they bar entry to and make visitors from ETA countries get on an immediate flight leaving - excluding obviously situations where someone has outright done something that forbids entry such as serious criminality. More common is they might go into more questioning and try to push to catch someone in an outright lie, but failing that, they kind of give them a lecture - which amounts to exactly nothing in meaning. You can still turn around and send the application tomorrow, you're allowed to, ahem, 'change your mind.'
Also note - there is no bar on someone entering with plans to apply for sponsorship, as long as they also have plans to make it a visit (unless their PR status comes through first) and will respect the law / terms of entry i.e. leave when they are no longer allowed to stay. But planning to 'leave when no longer allowed to stay' does not preclude one applying for and receiving extensions, being allowed to stay under some other status, etc., etc.
[This is technically called dual intent but I don't like to use the term and suggest others don't use it with CBSA]
* Someone will counter 'they know a case.' I'm sure there are cases. I'm saying it's fairly rare.
