We are cruising with Carnival and stop in Key West 10/21. We have two girls, 9 and 7. Any suggestions for how to best spend the day? They are pretty adventurous kids. Is it worth trying to go to Smather's Beach or other beach area? Also, any recs on a good seafood place for lunch?

It is not worth a beach visit... use your time to ride the Conch train around the island and walk up and down Duval St...
My kids always liked the Conch Republic Seafood grille... its big, open and near the water... you can also visit several of the museums if the weather is iffy... have fun

They may enjoy the butterfly museum as well.

http://www.keywestbutterfly.com/

Thanks, y'all!

Would they enjoy Blue Heaven, a casual restaurant under the trees where live chickens wander around your feet and there is live music?

A number of the more popular bars are pretty raunchy, and the action is visible from the street. You may be okay with this for the girls or you may not be, so you need to know in advance.

You could do a glass bottom boat tour, do the Conch Train, or go to the Mel Fischer treasure museum. There's also an aquarium, but we thought it was pretty small and outdated. Not sure if they've updated it at all.

Here's a very minor cautionary note. Fantasy Fest (Key West's version of Mardi Gras) begins the day after you'll be there, and it's entirely possible there will be some early revelers.

If I were you, I'd stick to the Conch Train or Trolley tour (the train tour is about an hour and a half), and otherwise limit your time on Duval. The Butterfly Conservancy is a great exception and I'd definitely go there. The other suggestions you've gotten are great.

Would definitely recommend the glass bottom boat tour and train ride. You will easily spend 4 hours doing both, along with stopping to eat. There's also a lot to do and see around Mallory Square. At sunset it's the best location so if you can head over there because it's remarkable. The square also has lots of performers and entertainers as well as little shops so it's worth a visit for sure.

If they're on a cruise, the boat has to leave before Sunset Celebration... can't block the view...

Thanks, y'all. We took the Conch Train and did the Aquarium. Both perfect for our family. Lunch at Alonzo's...very good. Wanted A & B but they are only open for dinner. Alonzo's is their lunch spot. I had the Coconut Grouper with Lime Aioli...excellent! View was very nice too. We also had breakfast at Two Friends. Sampled from everyone's plate...Crabcake Benedict, Shrimp and Crab Quiche, Banana/Walnut Waffle with Praline Syrup. All very good.