If you reduce the amount of notes you need slightly further, you will be able to avoid the monotony of playing in C Major, for every single piece you play. Many well known melodies (particularly in various types of folk music) only use Pentatonic scales, which have only five pitches. The C Major scale ("white-notes" on the piano), C D E F G A B, actually contains three Major Pentatonic scales:
- C D E G A - C Major Pentatonic
- F G A C D - F Major Pentatonic
- G A B D E - G Major Pentatonic
(It also, therefore, contains three Minor Pentatonic scales, and the other modes of the Major Pentatonic scale; but it might not be necessary to know about that just yet.)
This webpage shows each of these Major Pentatonic scales, as being part of the C Major scale. It then goes on to give a number of examples of melodies using pentatonic scales. (Although these melodies are written using conventional notation, a helpful diagram in the middle of the page shows these pitches with their associated letter names and solfege notation.)
As other comments and answers have pointed out (here and with your other question), you can transpose pentatonic melodies that are not in one of these keys, in just the same way as is possible with Diatonic melodies; however, in this case, you have the option of transposing into any of the three keys listed above.
Having the possibility of three different keys, to play pentatonic melodies in, helps you in one very important way, too. Some tunes may have the tonic as the lowest note; others may have notes both above and below the tonic. In the latter case, it may not be possible to play such a melody using C Major Pentatonic on your "virtual-piano", but it may be possible to play it in one of the other two keys. (As, I'm guessing, your virtual piano has only an octave range.)
There are many resources available online with pentatonic melodies. On the first Google page alone I found this, this and this. (The first is a list of songs, requiring you to do a little more research of your own, to find sheet music for these tunes; the third is a page linking to resources on many other websites.)
Finally, a few thoughts about the way you are intending to play music (which echo the answers and comments elsewhere on this page).
- You will be very restricted as to the music you can play, if you only use the notes of a C Major scale.
- Learning to use conventional musical notation will open up a world of new music (and musical understanding) to you; using letter-names is very restrictive (and restricts the resources you can use).
- Playing single line melodies on your "virtual-piano" will do very little to help you learn the technique used to play the piano.
- Using a "virtual-piano", you are missing several fundamental aspects of piano music and playing: playing with both hands (which creates chords, music in more than one part, harmony…); dynamics (louder and softer notes); and, of course, music with the full chromatic range (which this post is all about, I guess!)
Although I initially thought about voting to close this question, I figured you have shown enthusiasm, for finding solutions to overcome a number of restrictions, in order to make music; this can only be a good thing! However, I hope you use this current experience with a "virtual-piano", to lead you onto playing a real keyboard or piano. If you can beg, steal or borrow even a keyboard with a few octaves of keys, you will (I hope), both be really excited by the extra musical possibilities it offers, and understand more fully the comments made by others on this page.