The question "¿Adónde viajas?" is a question to know which PLACE it is going to go to.

In this case, since it has the interrogative "adónde" (where) we can deduce what it is asking.
I leave you the interrogatives in Spanish, which are used a lot to start the questions:
- ¿Qué? ↝ To ask for an explanation.
- ¿Quién/es? ↝ To ask about people.
- ¿Cuál/es? ↝ To request a selection within a group.
- ¿Cuánto/a(s)? ↝ To ask amounts.
- ¿Cómo? ↝ To ask the way.
- ¿Cuándo? ↝ To ask about the time.
- ¿Dónde? ↝ To ask about the place.
- ¿A dónde/Adónde? ↝ To ask about the place (must indicate movement).
- ¿Por qué? ↝ To ask the cause.
Curio: Both "adónde" and the two separate words "a dónde" are valid in the Spanish language.