There are lots of free printables for journaling cards that fit into those slots.
If you like, you can get some patterned paper to cut into filler for some of the slots. This breaks up the page a little, and also you can start a new "subject" on it's own page without leaving a hole, or the back side of another picture.
I'm with you on simple. My 12 x 12 pages have minimal decoration, I'm all about telling the story!