Books definitely! Linking to Amazon US for the most part, but can probably find many of these elsewhere.
Textbooks (ones I have personally used and liked):
- Genki 1&2, or
- Minna no Nihongo
This page has a bunch of alternatives: Choosing the Best Beginner Japanese Textbook For You
Reference books (useful at low or high levels):
- https://www.amazon.com/Handbook-Japanese-Patterns-Teachers-Learners/dp/4874246788
- https://www.amazon.com/Guide-Remembering-Japanese-Characters/dp/0804820384/
- maybe a “小学国語辞典” (JP <-> JP dictionary designed for elementary school students, often contain cute pictures and easy to understand definitions)
Other resources:
- https://www.whiterabbitpress.com/kanjiflashcards – amazing quality physical flashcards. Obviously not NEEDED if you’re sticking to Wanikani for your kanji studies, but definitely make a nice gift. Associated poster is also pretty cool to track your progress, a nice gift in it’s own right, but shipping cost is high if buying just that, so maybe try to order other things from their site (OMGJapan) as well. The site also sells a lot of textbooks and graded readers, so could definitely build up a list.
Fun books:
- https://www.amazon.com/Japan-Kodansha-Bilingual-Japanese-English/dp/4770021739/ (bilingual, side-by-side pages of Japanese and English translations of folktales)
- Buy Japanese Language Books: Great Selection & Worldwide Shipping – Tagged "Product Type_Graded Reader" – OMG Japan Graded readers (linked to OMG Japan site because it has a good collection, but may be able to find them cheaper elsewhere)
- any books/manga from the Absolute Beginners Book Club // Now reading: Hunter × Hunter list
- any slightly more difficult manga that you would be really interested in (can often find whole series for sale used on ebay for pretty reasonable prices, so if owning all of テニスの王子様 for example would motivate you, go for it!)
Anime or drama DVDs or Blu-ray could also be good gifts, if that’s something you like.