De qué

As you know, the usual response to gracias is de nada, you're welcome, it's nothing, think nothing of it. However, you may also hear por nada or no hay de qué. This latter reply is often shortened to a perfunctory de qué o no, de qué.

Sí

When service personnel are thanked, they may respond with a perfunctorysí, something along the lines of 'just doing my job'. Hearing sí in this context may seem bizarre to you, but it is simply a formulaic response, neither curt nor extraordinary in any way.

Gracias a Dios

When saying gracias to someone for having cooked you a meal, you may hear the response gracias a Dios. Presumably, God is the one who has provided the food.

¡Qué detalle!

When thanking someone for a gift or any kind gesture, you can say ¡Qué detalle!, How thoughtful! This is an especially appropriate reply for an unexpected gift, something small, un obsequio.