There are a multitude of great places in CT, especially on and near the coast. However most people don't really give specific locations up, you should google search old maps of your town. However, I'll drop a name and an example for you. I'm going to be metal detecting Briggs High School here in Norwalk, which hasn't moved since the 1930's
Also if there are any street or sidewalk tearouts (which, naturally there are going to be somewhere), see if you can find them on old maps. West and East avenue have been around forever in Norwalk, so whenever I see road work I tend to pop up around there. I MD'd a sidewalk tearout that was off of another old street. I did hit a lot of rocks, and found beer cans tossed by construction workers, but I did end up finding a huge cast iron piece dating from the early 1900's. Also, unless explicitly said, you can hunt most old parks and SOME historical sites, with the exception of battlefields, cemeteries, or near memorials. Private historic areas are alright, for what I know, as long as the property owner is okay with it! Also city/state parks that are National Historic areas are usually alright too, there are a lot of trips to those areas.
You should use Google
just asking removes the fun of having to research and gather all of your data! People will not usually share their prime locations. However, I will give you a few locations, but in the future you should research your own!
Samuel J. Preston Elementary School, Harrison, NY
Calf Pasture Beach, Norwalk, CT
Waveny Park, New Canaan, CT
Salmon River, Hamden, CT
EDIT: Just noticed that this was a dug up post, however, the information I provided is still current!