There are wonderful duets for other instruments too! The piano is not the most sociable instrument, actually, so if you think your daughter might like to be part of a school orchestra or band, a string instrument or wind instrument would be good for that.
The piano is wonderful, but it tends to be more solitary, I think. The piano or guitar give you the possibility to play more music on your own, because these are the instruments which play chords, harmonies, and counterpoint along with a melody. Other instruments tend to play one line most of the time- so good for playing tunes.
String instruments, especially violin, are hard to begin with, as it takes patience to learn to hold and create sound skilfully on a string instrument, but I think they are the most versatile and rewarding in the long term because of the incredibly varied repertoire. To me the very best thing about playing music is the chance to play in a small chamber music group, and string instruments just have the best chamber music! Also, you can play in an orchestra, solo, jazz, fiddle- so many options- and because there is a need for lots of strings in an orchestra (and just a few flutes or clarinets), there are often more opportunities.
I am a string player, so very biased. I guess I would recommend the cello if I had to choose, but only if you fall in love with it...