Premier is another company that does land tours. They work with a lot of the non-land-tour-operating cruise lines to offer combined packages; tours include rail, bus, hotels.
I was a tour guide years ago for Royal Celebrity (a combination of Royal Caribbean and Celebrity cruise lines), who operated the land tour portion of the above cruise lines. The tours were pretty solid; Premier purchased Royal Celebrity a couple years ago and merged the two companies, so I'm assuming (hoping?) they are still high quality! Just make sure to book some excursions as that's the best way to get out and on the ground.
(Also, just renting an RV and driving around Alaska is pretty amazing, and maybe book a round-trip on the train to Denali in addition, SO gorgeous.)