I've visited Skagway independently twice, once with car, once on foot.
By car, I boarded the AK state ferry in Prince Rupert. Alaska Marine Highway does not have a great website, but I believe that Prince Rupert should be an available port in 2024, I see it should be once/weekly on the winter schedule and don't see a published schedule beyond. But Prince Rupert could be a nice option for you if available. As Dougwood pointed out in his post, there are state rooms available on the ferry boats. It looks like they aren't selling summer season tickets yet, but I'd be alert for it and pounce immediately when they post.
On my car trip, I got off for a couple days in Ketchikan and then reboarded for Skagway. Ketchikan to Juneau was a 24-hour overnight hop. But it's only about 6 hours on the ferry from Juneau to Skagway. On my carless trip, I flew into Juneau with enough time to reach the ferry terminal a few miles north of town, then arrived in Skagway in the late evening with no overnight stay on the boat necessary.
MV Matanuska en route Juneau to Skagway 26 Jun 2011
I'll second John C's suggestions for both steam and and a Carcross trip (the reopened section along Lake Bennett is breathtaking... plus you may see some mixed train switching). Hiking tickets are also available, where you ride the train to and back from a couple trailhead options. I took a Denver Glacier trip; I was in tiptop trail-running form, so I saw the glacier & returned to track with abundant time for railfanning. The old D&RGW Rotary ON is about a mile downgrade partly-buried as riprap, so I paid a visit. The Denver trestle is right at the trailhead.
108/104/101 lashup, Denver trestle
Also at the Denver trailhead is an accommodation option: a
caboose cabin! During the day there is no privacy, with hikers on site and cruiseship tourists gawking from every traincar, but at night it would be just you and the
bears. This is precisely when it's nice to have a metal tent.
There may be cruise packages that include both Whitehorse and Skagway, but again you would have more flexibility to visit friends on an independent trip. I proceeded to Whitehorse on both my trips. By car, the drive is lovely but not as nice as the train ride to Carcross. The road reconnects there; for a group with car traveling to Whitehorse, it could be an option for someone to drive the car to meet train passenger(s) there. Without car, there are thru-tickets to Whitehorse with the final leg from Carcross by bus.
A car in Whitehorse puts Dawson City within range of a rushed-day/leisurely-overnight trip. The 3 locos there are certainly worth a visit. You could return to southwestern Canada via the Alcan Highway if you don't want to retrace the ferry route. Or you could do the loop counter-clockwise (Whitehorse first). For the no-car scenario, as a Canadian you may find Whitehorse to be a good airport to fly into or out of.