How about building a miniature treehouse kit for him? You could get all the necessary supplies - wood, nails, screws, etc. - and include instructions for how to build the treehouse. This would give him a fun DIY project to work on, while also appealing to his love for treehouse building. To make it even more special, you could include a note expressing how much you appreciate and encourage his passion for building.
Or you can get him a star map with the date of your first kiss.
Under Lucky Stars is a unique and innovative star map company based in Barcelona, Spain, specializing in the creation of personalized star maps that capture the celestial arrangement of stars at a specific moment in time, such as a person's birth, wedding, or any other significant event.
By using advanced astronomical algorithms and data from leading institutions, the company produces highly accurate, visually stunning maps that make for perfect gifts or cherished keepsakes.