I have about 3.5 days in Seattle from thursday PM to sunday night. Too late (expensive) to buy air tickets. Wondering if I can make it to Banff and back and make it a worthwhile trip in 3.5 days. It looks like 10 hours of driving each way. Any tips will be much appreciated.

Forget it. My take is "No." You would essentially drive nonstop only to turn around and come back. We usually break that drive into 2 days each way. Best case, 10 hours without factoring in any stops, and hoping that you don't get stuck behind any campers or large trucks on the road (these are not U.S. style freeways). As a single drive, you should probably count on 12-14 hours, depending of course on where in Seattle you are starting from. And then last-minute lodging will be challenging.

As an alternative, consider heading up toward Vancouver. You could easily spend time there, up at Whistler, on the Sunshine Coast, even catch a ferry across to Victoria. Or look at the many options in Washington, such as the Olympic peninsula.

Agreed for all the reasons above. Not worth it. Way too far for 3.5 days. Banff is the most sought-after destination in Canada right now. I wouldn't show up spontaneously at the height of summer crowds. You'll be dealing with full parking lots and relying on shuttle buses to access the main sights, assuming you can get a hotel.

Vancouver Island and the Sunshine Coast are also great alternatives for a closer getaway in Canada, but I wouldn't necessarily plan travelling anywhere with a ferry without a ferry reservation this summer, especially if you're travelling around a weekend, or you'll risk waiting for hours and hours in a parking lot in a ferry terminal for the next available boat. Vancouver Island and the Sunshine Coast are busy places right now.

If you've already visited Whistler and you're itching for a road trip to a mountain destination, have you considered the Kootenays? This is the part of BC north of Idaho. Nelson is a phenomenal mountain town on Kootenay Lake with historic architecture, fantastic restaurants, breweries, shops, and cafes, and is worth a few nights and would be a fun home base. It's 7.5 hours non-stop to get there from Seattle. Rossland is another town in the vicinity - much smaller, but also historic - 6.5 hours away. You could combine the two in one visit, or just pick Nelson.

Here's the Nelson tourism website: https://www.nelsonkootenaylake.com/
Here's the Rossland tourism website: http://tourismrossland.com/

You might do better to opt for a Washington State-only loop beginning with Interstate #5 north to the North Cascades Highway... then to Winthrop, and eventually south and looping around Mount Rainier via Yakima and Chinook Pass. Such a trip seems a grand way to fill 3.5 days without the hectic pace you're considering.