phi phi

You can get the ferry there and back in the same day but unfortunately you would only have a little time to look around so not really a very effective day trip. Even the speedboat day trips to Phi Phi do not actually spend that much time there, usually stopping for longer at Maya Bay and some of the other areas around Phi Phi.

If you want to actually spend some timet here you would need to either hire a speedboat to take you there and back, or do an overnight.

Agree with #1 - The ferry takes about 2-3 hours or so each way from memory which means as soon as you dock you will need to turn around pretty much.

Speedboat tours are the way to go for a day trip, but I would highly recommend staying on Phi Phi for at least a couple of nights especially if you are weighing it up against Phuket.
I remember my first time in Phi Phi I was only planning 3 or 4 nights, then all of a sudden after 11 nights I looked at the calendar and realized I had to go and catch my flight!
The full day trips out of Phi Phi take in a great amount as well.

A lot of the Phuket daytrips to Phi Phi are done via the big ferries - but they have a pretty tight schedule once hitting the island. Like across to Phi Phi Don for The Beach and birds'-nest cave, then to Mosquito island for beach and snorkel, then maybe a snorkel off the east coast, then maybe lunch and a bit of shopping in town.
To get to these places some operators offload their customers onto longtails etc - in other cases the ferries unload Ton Sai bound customers at the town pier and then continue on to those destinations.

But yeah, if you wanted to do your own daytrip whithout being herded around you would find it hard to see much in the limited time.