1. You aren't going to drive from Seward to Denali and see anything in one day. The drive is very far.
2. In Seward: Do a fishing charter during the silver salmon derby. But, make sure you verify that the fish are still in schools and haven't broken up to start running yet, or you won't catch nothing and should be by the river heads on the shore instead. (half day fishing is like 150 pp)
3. I fee like the glacier cruises in that area mostly leave from Whittier, which is like a decent drive from Seward. There may be some from Seward also, I dunno. A decent glacier cruise will run you like 100 pp also.
4. Hiking near Seward. A cannot miss is the Exit Glacier hike. Pretty strenuous up to the top, but the views of the Harding Ice Field are amazing, and you can walk right up to (onto?) it. Bring a sweatshirt in your backpack on the way up even if its 100 degrees because it is cooooold up by the icefield.
5. Don't forget to include for the RV docking fees at the campgrounds you'll be docking at. I don't know anything about RV's. But, I do know there is a really great campgrounds on the road by the Exit Glacier (10 min out of Seward) which IIRC is free for tenters, and a moose walked right though the night I stayed there.
6. There is a guy who makes hand made knives on the road down to Seward. His shop is called IRBI. He's cool, stop by and shmooze with him.
I have lots more info about the area, but my trip (3 yeshiva guys) was probly a bit different than what you want to do.