There are a number of excellent published arrangements for piano of this well known Lied (song) by Schubert in the public domain. This video features an arrangement by August Horn. You can play the arrangement as written or add your own ideas as you go along, as many performers do today.
The sheet music is free to download from the IMSLP Music Library.
*Note: The PDF you will download from this link is a complete album called "Schwanengesang”. On the main page select the "Arrangements and Transcriptions" tab and scroll down to "For Piano solo (Horn)" the famous Serenade is page 15-17.