Hey Chris, ....There are lots of lights to see in CT, RI, and MA. I see you've already received some tips. Stonington to Watch Hill is a fairly short ride. Once you're in RI you should go to Pt. Judith, then over to Jamestown via the (Jamestown Verazzano bridge). There's a rusted and disgusting looking light under the north side of the bridge. You can also see Dutch Island Light from the bridge. Then go out to the southern most tip of Jamestown to view Beavertail Light. Then head back to the Newport Bridge ($2 toll). You can see Rose Island Light off to the south. Go over to Goat Island (via the causeway) and you can see the Goat Island Light and another view of Rose Island Light. Come back over the causeway and head south through the center of Newport. Look for Thames Street and follow that along the harbor until the harbor ends and you bear to the right to go to the Ida Lewis Yacht Club. Lime Rock aka Ida Lewis Light is right there. When you leave Ida Lewis continue to your right and work your way around until you see the sign for the Castle Hill Inn. Drive up to the Inn and go way to the end of their parking lot away from the Inn. Castle Hill Light is just a short walk through the woods.
No you need to get out of Newport and back to Bristol RI. Just before you get to the Mt. Hope Bridge there is a scenic overlook on your left where you can view Hog Island Shoal Light in the distance. You can also see a very small light called Prudence Island Light over at Sandy Point on Prudence Island. When you cross over the Mt. Hope Bridge going towards Bristol, take your first right (kind of a reverse turn) and follow that down under the bridge. You will see the Bristol Ferry Light and Hog Island Shoal in the distance. When you leave there, you can stay right on rte 136 and that will take you to rte 195, go east. If you feel like it, you can drive out to Sakonnet for a distant look at Sakonnet Light. If you continue on to New Bedford, and get off one of the exits that takes you to the waterfront you will see, Clark's Point (at the fort), Butler Flats, and one other who's name escapes me at the moment. You can also cath a glimpse of the New Bedford Lightship and one of the Nantucket Lightships right next to the USCG Base. Get back to rte 195 and find rte 6 east until you get to Mattapoisett and drive right up to Neds Point. Stay on rte 6 east and you will come to Wareham where you will see another Nantucket Lightship on your right as you pass over a small river. Continue on rte 6 east and you'll come right to the Cape Cod Canal.
I wish I could give you some real specific instructions but we're in the middle of packing to leave for another vacation tomorrow and I'm a little rushed and tired. I hope you have a wonderful trip and adventure seeking out these lights.
P.S. There are more RI Lights to see but they are a little hard to find and not that accessible.
Bob