Paint (or gel) markers are great for adding details on top of a background, whether it's an elaborate drawing or just a simple pattern. I especially like to use light coloured markers on dark backgrounds.
I really love my Gelli plates (even if I don't use them often enough). They are so versatile, allowing you to create all kinds of wonderful effects, whether you're just using colour, or adding stencils, stamps etc. You can print single sheets of paper that can be glued into your art journal (or bound into a journal). With a ring bound journal, it is also very easy to print directly on to the journal page to create a great background.
I found an old, and as it seems unused, Polaroid camera in a second hand shop some time last year, and, as far too many of my art supplies, it has been sitting ignored in the book shelf for far too long. But they're such fun to use. Capture a meaningful place or event (like your studio, where all the magic happens) or record a special moment (like the arrival of my new and impatiently awaited reading chair) and stick it on to a background to turn it into a special memory. Of course any other photo will work perfectly too.
Of course you can stick all kinds of things on to your journal pages. Patterned papers, little drawings made on index cards, a piece of scrap paper that you used to try out some new pens or markers etc.
I love these Japanese (book binding) papers, the colours and patterns are so beautiful. I like to use them to cover a whole page to create a background, and also to glue them on to the covers of an art journal. And last but not least, a simple cardboard tube makes a great stamp for adding circles or bubbles, either to add on to a background, or to create a background. Those little plastic caps that protect your newly bought brushes also make great stamps for small circles in different sizes, so remember to keep them.
I had so much fun creating these pages, and I can't wait to fill the remaining two or three pages. I've re-discovered so many tools and techniques that I haven't used in ages, and it I enjoyed finding so much inspiration within my studio. But now I really have to tidy up my studio. And this time it's not an excuse for putting off making art, but to make space to get to all my art supplies more easily.