Does a Canadian need a visa to pass from Cambodia into Laos and then back?

Mark's answer covers most parts so I'll throw in a couple of notes from my experience:

• The on-arrival visa is a single-entry visa; the same visa won't allow you to get back from Laos. You could get an ordinary visa or a business visa (the type 'E' that Mark refers to) but by default these are single entry, and you need to get an extension to get multiple entry status. As you said you're there for two weeks, you probably would find it less hassle just to get visa-on-arrival.
• There's a second kind of visa-on-arrival known as E-Visa (not to be confused with visa type 'E' which is a business visa) which is a sort-of pre-authorisation; not really needed though as you can get one at the airport anyway. An E-Visa can only be used for entry at select checkpoint although exit is allowed from all checkpoints.

Now for the Cambodia to Laos bit. I asked a question about this earlier here. If you want the easy way out you can look around for travel agencies which will arrange this for you. I wanted to do it DIY. Although Siem Reap is technically closer to Laos then Phnom Penh is, I didn't find any bus operators operating buses directly at the time. The closest border town is Stung Treng but for some reason it's easier to find buses to this town from Phnom Penh that it is from Siem Reap. Once at Stung Treng, you need to go on further on unorganised public transport.

Although visa-on-arrival is technically available for Laos at border checkpoints, when I visited this year a couple of months ago I was advised by the owner of a hostel I stayed at in Phnom Penh that they were no longer issuing them at the border and you'd need to get one in advance. I'm not sure whether this was actually true or still is, and I don't think it was a scam because he didn't direct any of us to any travel agencies to 'process' the visa anyway - he gave contact details for the Lao Embassy in Cambodia. Apparently it doesn't take more than a day or two to get a Lao visa in Cambodia itself.

Take a call on whether you want to get it before you begin travel, or get to Cambodia, find out whether they are still issuing at the border... and if not then get it in Phnom Penh.