Where to Vacation

The Dad and I are trying to plan a summer vacation for just the two of us. We haven't been on vacation for a number of years, so we are out of practice. We are checking out sites, but have not decided on any. Where should it be--Niagara Falls, Arizona, New England, Gettysburg, The North West states, Hawaii, Alaska, London? All are viable options, we just need to decide. Any suggestions?