My husband and I will be traveling to Thailand in November (for the first time). We plan to spend two nights in Bangkok, head to the beaches for five days, and return to Bangkok for the final night (which is, coincidentally, the Koy Kratong festival) before departing.

I've been researching the beaches, and I think I've decided to head to the western beaches rather than the eastern gulf (because I'm really digging the rock formations). I was thinking of spending two nights in Phi Phi, two nights in Ko Lanta, and one night in Phuket. Am I being overly ambitious with our travels? I understand that the flight to Phuket, taxi to ferry dock, and ferry ride to Phi Phi will probably take half of day. I just want to get the most out of our trip, but of course I don't want to feel like we're traveling the entire time.
Thoughts?
If we decided to ditch one of the three, would it be easy to do a day trip to Ko Lanta from Phi Phi or v.v.?
Would it be easier to take a ferry from Ko Lanta to Krabi to fly back to Bangkok?

We're looking for beautiful beaches, a little scuba, and some laid back fun in a romantic setting.

Thanks in advance for any advice!