I feel guilty that I never kept a baby book for DD (port-partum issues got in the way) and didn't even think to get one for DS, but I see that there are lots that start at 3 y/o as well. Do you keep a memory book (baby book/school memory book, birthday book etc) for you kid(s) and if so which one do you use?
Re: NWMR: memory books
ETA: Or basically I could have said ditto to @cjcouple
i also print digital photos regularly and put them in a photo album.
thirding the writing-down-cute-quotes idea. i would just get a nice blank book-- a scrapbook-- and keep track of funny things your LO says and put in some of your favorite pictures and other mementos.