You have two main options to make it fit after using scale: pad or crop. Take a look at these examples and the documentation for each filter.
pad
This will pillarbox the image.
ffmpeg -i input -vf "scale=-1:720,pad=1280:ih:(ow-iw)/2" output
A more generic command that will work for all input file aspect ratios will use force_original_aspect_ratio=1 as an option to scale:
ffmpeg -i input -vf "scale=w=1280:h=720:force_original_aspect_ratio=1,pad=1280:720:(ow-iw)/2:(oh-ih)/2" output
crop
This will cut off the top and bottom.
ffmpeg -i input -vf "scale=1280:-1,crop=iw:720" output
A more generic command that will work for all input file aspect ratios will use force_original_aspect_ratio=2 as an option to scale:
ffmpeg -i input -vf "scale=w=1280:h=720:force_original_aspect_ratio=2,crop=1280:720" output
Use your player
If you don't want to bother with re-encoding, then any player worth using should allow you to do this upon playback. Example using ffplay:
ffplay -vf "scale=-1:720,pad=1280:ih:(ow-iw)/2" input
Or see "Video Effects" in VLC.
