I am hoping/presuming if you are going to Drumheller that you'll visit the Royal Tyrell Museum (dinosaur museum). It's pretty cool (and not much else to see there other than hoodoos).
Second vote for Banff. If you're going to come all this way you have to make it to the Rocky Mountains. I would highly recommend heading to Banff and Lake Louise (only another 45 min past Banff). There's also Waterton Lakes National Park in southwestern Alberta which won't be that far from Lethbridge (it's in the mountains too). Heck, if you're going that far it's only another 7.5-8.0 hours to Vancouver and the Pacific Ocean for that matter