For a beautiful keepsake, why not bring a Shuuin stamp book to record each temple you have visited.
Shuuin’s books are beautifully made with cute cloth covers and approximately 20 pages of accordion paper. They are sold in almost every temple in Japan for around 500 yen. Buy one at the first temple you visit, and then you can fill the pages as you travel through Japan.
Each temple has a calligrapher who, for a few hundred yen, will stamp your book with the temple’s red seal in vermilion and add the temple’s date and name with skillful touches of ink. The calligraphy is in Japanese and it can be quite difficult even for Japanese speakers to decipher, so you may want to keep a separate list in your language to help you remember which page refers to which temple. But the result? A book full of calligraphy to document the temples visited on your trip to Japan.