Manches Iles Express out of a nearby port. There was a quick stop in Jersey and then the boat continued on to Sark. We arrived around 10ish local time (UK time) and the boat left Sark at 7pm local time so a full day to explore! Our round trip ticket on the boat came to around 47 euros per person. The other way you can get there is to fly, I think most of the flights leave out of Saint Malo though.
It is a tiny island and with no cars allowed, only tractors, horse-drawn carriages and of course bikes! I had take a look at some of the different bike options before leaving and found a good deal at A to B Cycle Hire which was 6£ per person (but they had an ad in the map from the ferry terminal which gave us a discount making a total of around 10£50 I think for both of us. After that we basically explored the island! Given that there are no cars there are also no paved roads! Even the "main drag" is a dirt road and felt like a cross between the Wild West and Victorian England! We enjoyed a fantastic cream tea, "toasties" for lunch and then a snack at the gardens in the afternoon.
I wouldn't say there is much "to do" there other than enjoy the wonderful countryside and views the island has to offer. There is a little micro-climate around the islands so they often have better weather than we do just on the other side.
It was a fantastic day, a great way to clear your head and to really feel like you've gone really far away (even though you can see France in the distance!) The people were really friendly and welcoming and all in all a great atmosphere on this little remote island. Highly recommend it!! The other thing that would be fun to do is island hopping between Jersey/Guernsey and Sark, one of the other operators from Normandy was offering a type of package and often times has special offers on, Vedettes du Cotentin if you're interested.