Lucky you, going to Sifnos! We've been several times and stayed in Kamares each time. Food wise, I would recommend Agianemi and Hotel Boulis's restaurant - both really excellent. There are several of the more traditional Greek tavernas too but these two stood out. Da Claudio is a great Italian if you fancy something different. The tavernas along the water's edge are lovely for traditional Greek food with a nice view - you can book tables at most of them too as they do get quite busy.
The buses are really easy to use from Kamares - Apollonia is worth a visit, it's very pretty and charming. It's also really easy to visit other beaches as well - Hersonissos is worth a visit and Vathy is pretty but quite busy in summer. Faros has always appealed to me but I've never made it there!
In terms of a boat trip, you could try Aegean Thesaurus, a Greek travel agency based on the island. They booked a boat trip round the island for us and it was brilliant. Lots of swimming off the boat, refreshments included and a lovely way to see the island.
Kamares itself has some lovely walks - the beach is quite long so it can be very enjoyable to walk from one end to the other and then out to the pretty church along the bay. There is a nice infinity pool at Seven Seas that you can stop off and use if you buy a drink, I believe.
There's also a great cocktail bar at that end of the bay as well. You could also catch the ferry over to Milos for the day if you wanted to explore further. We've not done that but friends did :)
Kamares beach itself is great - sandy and calm, but fun when the ferries come in, a bit of a buzz and some waves and then it it dies down again as the people disperse. Take some clips to hold your beach towels on to sun loungers - the meltemi can be strong.
You have to pay for loungers on the beach at pretty much all the locations (rather than just buying a drink) but the prices do vary so worth looking around.
In terms of snorkelling, Kamares isn't brilliant but you will see quite a few fish just swimming around and the occasional octopus. If you head down away from the harbour along the beach, there is better snorkelling towards the end.
I've never been to Syros but would be keen to hear what it's like! You could have a look at Matt Barrett's Travel Guides and Facebook group - hea a Greek American but is very knowledgeable about all the islands and I believe either comes from Syros or his wife does, so he shares loads of info about it and photos etc.
We're off to a different island this year - but would very happily have gone back to Sifnos. Enjoy!