Use the phrase "that would be ___" to accept someone's offer. For example:

A: Would you like something to drink?

B: Sure, that would be great.

A: I can watch your kids for you if you'd like to go out for a few hours.

B: Really? That would be wonderful!

You can use a variety of words that mean "good", including "wonderful", "amazing", "lovely", "fantastic", "neat", and others.