If you like radishes, they're easy to start from seed. You can get the seeds at Dollar General, Walmart, almost any hardware store, etc, and plant according to the instructions on the package. If you don't want to bother to thin them, and you've got enough room, just plant them farther apart than recommended. They can take a frost. In MA, unless you are up in the hills, you should be able to plant some now or very soon. Summer squash will have to wait until warmer weather, but easy to start from seed. They like lots of sun. Get the bush type for easier management. Ditto for cucumbers, zucchinis.
Peppers, tomatoes, I would recommend you buy as well-started plants, and don't plant them until the ground is warm. Tomatoes can be buried deep -- strip off any leaves that would touch the soil first. These are sun-loving plants.
If you've got ample room, pumpkins are fun and not super fussy. They like to be planted in warm soil with lots of compost in a spot that gets sun most of the day. You can plant the seeds directly in the garden, but I've had the best luck starting them in a container in an indoor sunny window up to a week before planting out. Use something like the plastic container strawberries come in the supermarket, poke extra holes in the bottom, some good potting soil, and viola!
With anything you start indoors or buy as a plant, harden them off before you plant them in the garden -- that is, leave them out for increasing lenths of time every day so they can get used to the sun and wind without getting scalded. For instance, start with a place they will get evening and/or morning sun for the first few days, then gradually move them into the environment they will be in.
Begonias... unlikely to come back unless you lifted the bulbs and stored them in a frost-free place. They are pretty sensitive to the cold.
You would still have time to start your own marigolds and zinnias (both easy, lots of varieties available online or in catalogs) celosias (tiny seeds, but pretty easy) petunias (tiny seeds, some easier than others).
What kinds of plants are you interested in? What kind of space do you have? Full/part/no sun? Do you have an indoor sunny window to start things in?