We visit the Alaska Wildlife Conservation Center on every trip. For me that'd be a must-see. FYI the drive from Seward to Anchorage goes along the Turnagain Arm, so you're following another CBer's recommendation below regardless. Exit Glacier is also an easy detour when just leaving Seward toward Anchorage, and worth it. If you're avid hikers think about hiking all the way to Harding Ice Field. One of the best hikes you'll ever do but definitely challenging--I think it's 4.5 miles uphill to get there and, of course 4.5 miles down to get back. A less challenging but still fun hike is just on the Whittier side of the tunnel. Like take the 1st exit after the tunnel, I think it's the Portage Pass trail. IIRC it's about 1 to 1.5 miles (also uphill) to a great view of part of Portage Lake and the glacier. Then turn around and you have a great view of Whittier and the glaciers on the mountains around it.

I'd suggest adding a Resurrection Bay cruise (great for seeing wildlife and great geological formations) and/ or a glacier cruise out of Whittier (better for seeing glaciers but not as much wildlife). Fishing outside Resurrection Bay is also great if it's the right season and that's your thing. If you're doing a cruise, though, Ketchikan may be even better salmon fishing, though a bit less scenic.

A couple restaurant recommendations for Anchorage: Moose's Tooth pizza. Also F Street Station--more a bar than restaurant but great halibut and chips. Double Musky in Girdwood is great but I think I prefer Jack Sprat's.

Again we've done that trip a number of times. Feel free to send a boardmail for details. BTW one of the few places you can rent cars in Seward is from Hertz, last I checked. And from Whittier I think it was Avis. Both are *very* small operations, so plan that early unless you pick the train instead.