Jenn, I've not done Florence by bike and frankly wouldn't do it that way, especially with young children. While I'm sure the tours TRY to use less congested streets, I've no idea how they get to the highlights without having to maneuver some heavy pedestrian crowds and/or vehicular traffic. It's just a much easier, enjoyable, less risky place to sightsee on foot than 2 wheels, IMHO.
With the little time you have, I also wouldn't do a boat cruise - aside from Ponte Vecchio, what's along the Arno isn't as interesting as other parts of the city - or a food class. Better to hit some of those interesting parts that figure into the Florentine story.
We are hiking the Duomo in the morning,
Can you explain a bit? The church is pretty close to the main train station (S.M. Novella) so if that's where you're arriving, you won't have much of a "hike". Hopefully you've pre-purchased tickets for the entire Duomo complex, and are aware that to climb the dome requires advance reservations? The interior of the cathedral is less impressive than the exterior, and some other parts of the complex (museum, baptistry) are more interesting. Anyway, covering all the bits can take some time.
With however many hours you have left in the afternoon, I'd do a self-guided walking tour of other sections of Florence. Most good guidebooks include suggested routes (Rick has one also available as an audio tour) and moving along at your own pace might be a good idea for your youngest children.
https://www.ricksteves.com/watch-read-listen/audio/audio-tours/italy
