"to" means you start the trip outside of Spain and end the trip inside of Spain. "in" means you start and end the trip in Spain, even if you need to travel there first (which isn't part of the trip).

to Spain 'IN' shows the place, 'TO' shows movement to this place. For example, I don't live in Spain. I 'm going to Spain next week.

go on a trip in Spain i think

You travel from somewhere else TO Spain (i.e. from England to Spain). But, if while in Spain, you decide to go visiting, say, Castile, you can say, "While in Spain, I took a trip to Castile."

Matt is right!