Azores Islands (Portugal) - itinerary help & tips?

Hi! I am planning to spend 7 days in Azores, but because of the many islands, I am quite confused about how I should plan my route / itinerary..

So... I will be there in the second week of June and my top priorities are:

Hiking to check out the lakes / calderas - Sete Cidades (Sao Miguel) & Flores. These two seem to be the most popular - are there other sites I should check out? As a beginner, is it safe to hike alone? (I mean safe from Nature...) Dolphin / whale watching - do the boats only set out from Sao Miguel, or other islands from which I can depart? Thermal bath - Terra Nostra (Sao Miguel)

Seems like most of these can be done on Sao Miguel, but I have also heard (from a friend) that if I wanna make the best of the Azores, I will have to do some island hopping... What am I missing out? Or, which other islands would be worth a trip to?

Other general questions I have: Would it be better to fly or take a ferry to visit other islands? Is it easy/convenient to travel around Sao Miguel on public transport?

Any other tips or recommendations you wanna throw my way would be very welcome!

Obrigada!