Apple, banana, grape, strawberry, melon. The first five.
Pear, kiwi, orange, plum, peach. The next five.
Thereafter you can try any combination - but not in a pie or in anything with sugar or pastry.
Or cream or custard or milky substances. This shall be the whole of the law.
This all sounds easy but in practice it is hard. As hard as unseasonal fruit. A particular dislike of mine. Keep it real and avoid the unexpected. Think hard about the benefits and digest your own answers.