Kenai Fjords National Park is pretty simple... Rent a car, drive to Seward and do a day cruise. You could also use the Alaska Railroad's Coastal Classic, but it'll be a long day: https://www.alaskarailroad.com/ride-a-train/our-trains/coastal-classic
You next easiest option would be Denali National Park. Use the rental car and allow three days. One to get there, one to drive as far into the Park as the road allows (probably mile 30) and then the 3rd day to get back to Anchorage.