Holidays to North-East usa?

Hi well i'm from the uk and dying to goto the usa i haven't been since 2 years and i love it like hell! I have been twice and each time that was florida but we can't afford that this year and we are going on a super long holiday next year to the usa but i want to go this year aswelll, so i've decided that i will pay for my ticket and i will goto the northeast instead of the southeast so that it'll be cheaper.. But i don't know much about the northeast where would be the cheapest and have the most luxourious accomadation and the best attractions i.e loads of theme parks? Is the northeast cold in the summer like 20c!? IS it cheap to fly there compared to the southeast like how much for four people? What is it like for malls etc? How much different is it to the southeast? My dad said about cape cod but is that warm?