I am coming to the US in August for a holiday, and have slightly stuffed up one of my bookings. I am arriving at DFW on AC at 4.30pm and have a 10pm flight with Qantas. AFAIK I cant check my baggage through, and have to wait around 2.5 hrs before I check in.
I'd appreciate some suggestions on how to pass the time. Hyatt has quoted $139 + taxes for a room (bit too much for me). Is there anywhere to leave my bag, and is there anywhere I can relax?

Since they're both international flights, you will arrive into and depart from terminal D.

Your best bet to store the bag might be the hotel bell stand at the Hyatt in terminal D. Most hotels will check a bag without question, and if there's any resistance, I'd bet it goes away with a $10 tip.

As far as relaxing...
$35 will get you into the Club at DFW.

If you don't want to spend anything, the best places to relax at DFW are the Samsung Travel Lounges. These are small, semi-open rooms, free to the public but underutilized, thus quiet. You'll find chairs that are much cushier than the ones at the gates, plus desks with electric outlets to work on your computer (wifi is via t-mobile for a fee). None in terminal D (they're at gates A29, B8, B28, C8, C27, E8, E31), but you have lots of time to to get there. The air-side Skylink will get you to any terminal in 10 minutes or less.

If your flight from Canada has customs pre-clearance (most do), then you'll arrive airside. But if you have a checked bag, you'll go landside to claim it, and if you have to wait to check in for the next flight, you won't be able to get back airside until you do. And there are almost no amenities at all landside in the DFW terminals. My advice there would again be to head to the Hyatt hotel, to the restaurant, bar, or even just the lobby.

If I have printed my BP off the internet, is it possible to pass through security, then come back out later to check my bag in?
I will have access to the Qantas lounge but probably not too early.

Yes, you can clear security more than once. I'd imagine this is relatively common, since DFW security lines are usually pretty short, and the only smoking areas are curbside, outside security.

Thanks for your answers, that's probably the way to go. I'll leave my bag at the hotel, go through security then come out later to check my bag.

You'll be in terminal D and if you have to spend 2.5 hours landside, I'd do it at the bar in the Grand Hyatt (which is open to all, not just guests). Heck, my co-workers and I often meet there for happy hour when we are all arriving back at DFW fairly close in time to each other. At that hour of the day, the lounge should be somewhat lively and its a nice place to decompress until you can check in.

One other note... if you are on the same itinerary or staying on OneWorld, you might see if they can go ahead and tag your bags at your origination point. AA does maintain a baggage drop outside of immigrations/customs for baggage recheck and if it is tagged, they can probably accept it there which would eliminate the need to wait around. That's always been the case for me.