Day trip, roundtrip Tokyo Shinjuku/Kamakura

Hi guys, we are doing a day trip to Kamakura on Sunday, and are trying to determine logistics - we would like to leave Shinjuku station before 9 am for Kamakura, then return to Shinjuku late afternoon/early evening. What is our best option for rail pass, and how do I buy it? i.e. which regional pass, or is this one we'd have to purchase onsite, and not in advance before we travel? Both from Canada with Canadian passports.
Thank you!