Whenever I have a group of friends that want to go paddleboarding and need rentals, I like taking them to Deep Cove Kayaking. One time my friends an I rented the Starship, which is a giant 4 person inflatable SUP. I highly recommend this for a hilarious forced team building experience.
Deep Cove is a beautiful place to paddle because of the views up Indian Arm. With the mountains towering over you, it feels like you've escaped the city completely. If you stay within the cove, you feel relatively protected from the currents and boat traffic in the summer. It's fun checking out all the fancy boats and houses and waving to all the hikers at Quarry Rock.
After going for a paddle, I like to grab a snack at some of the local shops like Honey Doughnuts.