How useful is furigana for most adults and teenagers? If I am going to print some text to give to backpackers, is adding furigana worthwhile? It's important to avoid misunderstanding, but I would think people coming from Japan to Spain would be fairly literate.

Furigana is basically not necessary because backpackers are not elementary school kids. Adding furigana to easy words can even be disturbing. Ordinary news articles written in Japanese have almost zero furigana. There are many difficult words which native speakers cannot read without furigana, but simple guidance text should not contain such difficult words in the first place. One exception is uncommon kanji proper nouns which can be very difficult. But since your text seems to be about Spain, this should not be relevant.

At any rate, I recommend you have that text proofread by a native speaker :)