Fifteen miles is a perfect excursion for your first open water experience. I don't see the need for a liferaft, unless you feel uncomfortable.... my daysails are always in open ocean and usually 15-25 miles depending on the time I have.
The difference for you, of course, is that you are exploring new territory. That is exciting no matter how far you travel and how far you venture off shore.
My biggest concern was always about getting lost in the fog. So.... a gps with appropriate waypoints, and charts. If you have a fancy chartplotter, that's very cool. But charts and a gps are a must.
A harness and tether.... and wear it... and stay clipped on especially if you are solo.
Next... vhf radio.... fixed mount and handheld if possible. Put it on the weather channel every so often.
Binoculars to help you pick up nav aids and recognize landmarks.
Auto pilot/tiller. The best $500 you will ever spend. Use it occasionally to give your self a break, check your navigation, use the head, make a snack, stretch out. While the AP is on is good time to scan the area with your binoculars.
Healthy batteries...... and a frugal approach to power usage.
Plenty of fresh water.
Extra fuel.... but 15 miles is not going to tax your resources too much.
A comfortable seat cushion, a throw blanket, towels, foulies and proper clothing.
Make sure all your safety gear is up to date, in case the coast guard wants to inspect you (that happens a lot around here).
Let someone know that you're going overnight.
Rich's suggestion regarding cockpit drains and lockable hatchboards is advised if you don't have a bridge deck. (although I must admit that my hatchboards don't lock)
Attitude.... do not linger near shore. Go out at least 2 miles. Deep water is better, The waves aren't as annoying.
Make sure the bilge pump is working.
Check the through hulls and tie the appropriately sized wooden plugs to them.
IMPORTANT...... take notes.... write down things of interest. Record your position every hour, mark down speed, course, time, sea and wind condtions, weather. Describe your feelings, comment on anything that seems interesting or important. For instance, spotting a particular bouy, how the boat handles in certain conditions, the quality of cell service, if any.... etc. Make note of your mistakes and what you would do differently. Take pictures. Create a little guide book for each leg of your trip. Use it for future reference.