The most excellent fast after Ramadan is God's month. al-Muharram, and the most excellent prayer after what is prescribed is prayer during the night. (Using translation from Muslim 1163a)
قَالَ رَسُولُ اللهِ ﷺ «أَفْضَلُ الصَّلَاةِ بَعْدَ الْفَرِيضَةِ قِيَامُ اللَّيْلِ وَأَفْضَلُ الصَّوْمِ بَعْدَ رَمَضَانَ الْمُحَرَّمُ»