We are in Key West right now. We have done the drive from Miami 2-3 times, and now we fly in and out directly.
The Dry Tortugas trip is great. Just be aware that it has become very popular. As of now, the first availability for two people is June 16, two weeks from today. So you would need to book it in advance. I'm not sure how many nights you would plan to stay in KW, but the Dry Tortugas trip would take up one full day. You have to check in for the boat at 7 am and you arrive back at 5:15 pm. Breakfast and lunch are served on the ferry.
As for KW, there are so many places to explore other than Duval St. As soon as you are one block off of it, the atmosphere changes. There are lots of neighborhoods with some amazing restaurants. It's well worth getting off the beaten path. Since you will have a car, I would say just plan on parking it at your hotel or B & B, and either walk, rent bikes, or use the Duval Loop bus to get around. We did the Hemingway House tour for the third time a couple of days ago and had the best tour guide this time. He was so passionate about the subject of Hemingway, his relationship with KW, and the cats! You may also want to add to your list the Truman Little White House, the Butterfly Conservatory, the Lighthouse, Fort Zachary Taylor park (just visited it for the first time yesterday and it was beautiful), the US Coast Guard Cutter Ingham which is now a museum and you can tour the ship, go to the Sunset Celebration in Mallory Square which is held very evening. Duval St. is usually busy and yes in the evening you can encounter people who have over indulged, but we have never had a problem. We were out til after 11 pm each night and never felt unsafe. There are some very cool local art shops and book stores to explore.
We have been here 8 or 9 times and we still have things to do and restaurants we want to try that we have not been able to get to!
Thanks for checking in with advice. Will we need dinner reservations? We aren't wanting fancy meals. We were going for a more laid back relaxed trip but now that it's less than a week away, I'm worried that we didn't make any.