A warm up exercise I use helps with both stretching and flexibility.
Hands one octave apart starting on C in unison using RH 1-2-3-4-5 / LH 5-4-3-2-1 play the following pattern: C, C#,D, D#, E and back down D#, D, C#. Then repeat the pattern in whole step C, D, E, F#, G#, F#, E, D then repeat with a minor third between notes C, D#, F#, A, C, A, F#, D# then repeat with a major third between notes C, E, G#, C, E, C, G#, E and then instead of landing back on the C land on C# and start the whole group again until you have gone through all twelve notes of the chromatic scale. Everything is to be played legato with as much ease as possible. (Start slowly.)
You can make the exercise really difficult by adding one last pattern to the group this time in fourths. (C, F, B-flat, E-flat, A-flat, E-flat, B-flat, F)
