My parents, as well as my husband and I, will be arriving in Anchorage on Wednesday May 25th around 4:30pm, and will be leaving very early Friday morning. We will take a train to Seward, where we will start a cruise that day on Celebrity (Seward to Vancouver).
That leaves us with 1 full day (Thursday) in Anchorage. I need recommendations for that day. Should we just walk around downtown, or should we rent a car and do a driving tour.
If we do not rent a car, any recommendations on hotel and their locations?
Which hotels downtown offer free shuttle to the airport and rail depot?

You could stay in town and walk around, however, my recommendation would be to rent a car and drive out to the small town of Eklutna Historical Park(http://www.alaskaone.com/eklutna) and then continue on to Palmer to visit the Musk Ox Farm (http://www.muskoxfarm.org/). We enjoyed both these experiences when we were in Anchorage on our own. You could also stop at Thunderbird Falls, which, if I remember correctly, we did first thing on our drive to Eklutna.

Another recommendation: visit the Anchorage Museum. You will be immersed in Alaska's heritage, native cultures, and wildlife. We found this a wonderful introduction to our first Alaskan cruise.

There is a botanical gardens on the outskirts of Anchorage (heading toward Seward). I don't recall the name of it, but it was a wonderful place to walk and see some of the native plants (including one called Devil's Club - don't touch the leaves, they're full of sharp spikes!).

I instead suggest you head to Seward and consider a top tour choice- a Kenai
Fjords boat tour. Excellent wildlife and glacier viewing, superior to anything on the ship. A rental car is the best option for your short time.

Anchorage is home to the largest float plane fleet in the world. We have booked a float plane trip and when we told them we were only there one day and had no rental car, they said, no problem, we'll come pick you up! Also, we plan to walk or bike the Tony Knowles Coastal trail.