I think several names on your agreed on list are fun - Darwin, Phineas, and Emmett certainly qualify as fun. and I really do like Henry as well.
My choices would be Henry Lowell or Phineas Edwin.
But you asked for other ideas so here goes.
names like Darwin: Devon, Rowen, Aaron
names like Matthew: Matteo, Matthias, Merlin, Hugh
names like Phineas: Finley, Phillip, Felix
names like Henry - Harley, Heath, Harry, Enoch
names like Emmett - Everett, Elliot, Rhett, Emmanuel
Names like Thomas - Theodore, Thaddeus, Thor, Thurston
Thank you everyone for your suggestions. I love the name Theodore, but my best friend just used it about 3 months ago, so if we did, it would have to be a middle name. Thanks for bringing up that Phineas and Tessa have a lot of "s" sounds together. That does make sense. I'm wondering if I'd be able to do "Finn" (not Finnian, Finnegan or anything longer)... I really like the name Ethan, and it wasn't on our list, but it looks like it's #3 right now, so for me that's too popular. I kinda do want to avoid another T name, just because we're definitely not going for a T thing, but I would use another T if it were the right name. I do like Thaddeus, but people often mispronounce it ("correct way" is with a short pronunciation, as in apple). I know Darwin sounds geeky... that's kinda ok with us. We'd rather the name has a connotation of "smart", than a less desirable trait.