For a one day trip, go to the Magic Kingdom.
For a seven day trip, go to all of the parks.
In between, it depends on how long you are staying and what you want to do. Are you also going to Universal? Sea World? The beach?
As for construction, there is never a time when work stops at WDW. Something is always being built, redesigned or refurbished. Right now, the major construction projects are at Animal Kingdom and Hollywood Studios, with a major attraction (Soarin') closed at Epcot.