Ear Practice

Just listen to a random melody and play it on the keyboard. Start with the key of C and three notes. Once you get used to it, you can change the key, increase the number of notes, or turn the keyboard sideways to widen the range. You can also play melodies you know freely. Even if you can't get the notes right, just listen to the root note and then hum it, or listen to it repeatedly, and you'll gradually be able to get the notes right. If you practice every day, you'll be able to play difficult keys smoothly.