I'm going to Boston next month and was wondering is it worth going to cape cod for just a day trip?What else is there to do for just day trips?

How long will you be there?

It could be if you don't mind the drive up & back and you like:
Exploring awesome beaches for free.
Tons of bookstores, gift shops & antiques
Visiting unique places like Provincetown
Sites like CC Canal; Chatham Light; Long Point
At the very least you could boast that you have been to CC and in you like it you can return when the weather is nicer.

There are however a lot of places a lot closer to Boston: Cape Ann; Salem; etc.

I would second Salem for a day trip. You can take the commuter rail right to Salem. It is pleasant to walk around and their are places to eat on the water, as well as the history of course.

I plan on being in Boston for about 6 days or so.How long does it take to travel down to cc?Is there trains or just buses?

No train, just bus. If you are not driving, the trip becomes much less attractive, in my opinion. You could take passenger ferry to Provincetown for the day and back, or stay overnight. But without a car, I would skip the Cape - to get to nice places on Cape, you need a car.