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 :)
Unless the write want readers to read the kanji in an unconventional way (e.g., 歳月｛とき｝) or if the kanji is non-regular use, i.e., 表外字 hyōgaiji (e.g., 蘇｛よみがえ｝る), there will be absolutely no need for furigana if the intended audience is adults/teenagers. Meanwhile, there is no reason to include difficult kanji anyway; these usages appear almost only in literary works.
Some difficult kanji is unreadable for some adult in Japan. For example taro aso or shinzo abe couldn't read 踏襲 未曽有 云々(toshu,mizou,unnnun). Japanese standard reader may dislike furigana. On the other side, furigana is useful for kids or foreigner.