Bailar salsa es maravilloso / maravillosa?

In this sentence, we are describing the verb phrase "bailar salsa" and not just the noun "salsa". 

La salsa es maravillosa. -> La salsa (feminine noun) + maravillosa (feminine adjective)

Bailar salsa es maravilloso. -> To dance the salsa (verb phrase) + maravilloso (masculine adjective)

This is because when the infinitive verb forms are converted to nouns, they take on the masculine gender. Hence, we can also write the sentence as "El bailar salsa es maravilloso."

Here is the lesson that describes this concept - Using Spanish infinitives as nouns with the definite article El