"She likes to play the piano." is more formal, so it's probably the better one to use. But both can be correct. "She likes playing the piano." is a more "colloquial" way of saying it.

she likes playing...一般表示经常,习惯性做的动作,而she likes to play...则表示不是经常习惯性的动作,只是有时,偶尔做的一个动作