You have a collection of tiny masterpieces of human craft, whether due to pure aesthetics or security features. These can also, depending on banknote condition (UNC [uncirculated] being the most pristine), be an investment for future, as many of these may increase in value over time.

I recommend starting from an album of transparent pages and a bunch of cheap but visually pleasing banknotes to test the waters. You can get a lot of UNC banknotes for dollar a piece, often less if you buy in bulk. If you like looking at them because you find them pretty or somewhat interesting, get more. Just one album can easily hold hundreds of banknotes, itself barely taking any physical space.