1. what is a cheap phone/device to get?
The cheapest device is the phone you already own, and given you're in Canada, it's pretty well guaranteed you have a GSM handset. Get it carrier unlocked.
2. what are good-pay-as you go USA data plans?
There really aren't any, especially for phones. Carriers don't like selling data only plans for anything but tablets/hotspots/modems in the US, and even these data plans aren't PAYGO so much as flat monthly charges for X amount of data.
This said, given your circumstances, your minimal usage approach, and your desire to basically find an "international" carrier,
KnowRoaming. It's an international roaming SIM sticker overlay for smartphones. You can basically toggle between it and your native carrier, only use what you need, and even facilitates call forwarding from your primary cell number while out roaming. Uses both the AT&T and T-Mobile networks Stateside, and KnowRoaming is based out of Canada. Fortunately, unused balance carries over for 15 months of idle time, the SIM sticker never deactivates, it works globally, and their US PAYGO rates for data are 10¢/MB, which is about average for PAYGO data through any MVNOs or major carrier offering PAYGO data access for mobile phones. They also offer "unlimited" data plans for the US at $8/day.
Alternately, there's
Truphone SIM for international roaming, but it replaces your existing SIM card instead, and they provide free US phone numbers. Similar deal to KnowRoaming rates, though better/worse depending on the country. US rates are 9¢ minute/SMS (both outbound, incoming are free) and 9¢/MB of data. Same AT&T and T-Mobile coverage Stateside as KnowRoaming. Balance never expires and no account maintenance fees so long as the account is kept active at least once every 40 days, IIRC (basically send yourself an SMS message on the Truphone number and swap SIM cards to receive it). Based out of the UK. If you want to simplify switching SIM cards going Truphone, look into a
Magic SIM dual-SIM adapter for your phone. They're a little pricier than the cheap generic ones out of China from Ebay, but you also get guaranteed functionality and a year warranty if you don't know what you're looking for exactly with the cheaper ones - which can be dicey on occasion.
That should get you squared, just let me know if you need anything else. Happy travels!