Doodling in a diary or journal.
Doodling ideas.  Creative doodling ideas.
Creative diary or journal ideas.

First off, you need to test the paper of your journal/diary with the inks you will be using to learn how porous the paper is, how it absorbs the ink, how long it will take to dry, does it bleed on the paper or onto the backside of the paper.

Second — make sure your pages can handle extra bulk if you are using a spine book.  If you are adding extra bulk to your diary/journal, it will fatten up your book and may stress the spine.  Keep this in mind.
If you are just doodling on each page without adding any other type of embellishment, no need to worry about the stressed spine.

Blank books (with lines or no lines) can be bought at book stores, card shops, gift shops.  An artist sketch book (spiral binding) would work fine too.
Yard sales, thrift shops, or the like may have new unused journals or diaries.  Make sure they are in good condition and if no wrapping, check the pages for liquid spots (coffee, wine, etc).

Here are some ideas to make your diary fun, interesting or interactive:

You may want to add/glue down things to every other page in your diary/journal to prevent too much expansion.

Learn how to doodle! 
Common doodling pens — Sharpies, Bic Markits (both good for doodling on photos and are permanent) Sakura Micron pens, regular pens,  gel pens, glitter pens. 
Most of these pens come in a variety of colors and pen tips from micro-thin to thick.
For dark paper: Uniball White, Sakura Souffle

Basic Doodle Technique
Note the pen angle (how you hold the pen) when you doodle.
Make sure you are not touching the paper so that previous lines smudge.
There is no right or wrong way to draw — let your art flow.

Draw little flowers, animals, stars, buttons, use lines, shapes, swirls, loops, curves, skulls, hearts, dots and dashes.
Lines and shapes can be areas to write in.
Mix it up with pen tips and ink colors.  Add on brush markers or colored pencils to your doodles either coloring them in or adding them as highlights to some of your lines.
Practice this on scrap computer paper or recycle the backside of paper or use a notebook as a “doodling log”, easy to take out when you are bored.
There is no exact art to doodling, it is a free form of art.  Imperfections make it unique and original to the “doodler”.  There are no mistakes!
Save them in a file folder if you drew something good and wish to repeat it later in your diary/journal.

The doodles in this example were done with a Sharpie medium point marker since that was all I had with me at the time.

Cut out a doodle, color it then glue down.

Make page border collages of images, or doodles.

Make “frames” or “borders” around your entry titles Just by drawing simple lines and add a heart or other item in the corners.
Frame an entire page by doodling along the edges.

Learn how to create different doodling alphabets to write and emphasis single words. Doodle numbers. (see list of helpful books below)

Paste down the first letter of the word on the page, like you see in storybooks — the “O” in Once is usually pretty ornate.  You can find these letter images on the web if you do a search for that letter.
Copy and print, then cut and glue.

Use stickers or rub-ons.  Stickers or rub-ons can come in a variety of subjects or even quotes or printed verses.  You can find these in a scrapbook store or one of the national chains that have a scrapbooking section. OR use your own choice of quotes, choose your font, and size, print out, cut and paste.
Smaller quotes, like one line quotes, can be glued to any side of  the page.
There are stickers that have a nice acrylic raised surface that look nice also.

Write little notes or quotes on notebook paper or colored scraps then paste them in. 
Prepare some notebook paper by using an eye dropper and allowing the drop of water to bleed out the blue or pink lines.  Let dry then use.
You can have a bin of interesting papers to write on then pick and choose.
Titles of pages can even go on the scrap slips.

Write on acetate, then glue down the backside with Diamond glaze on some scrap of exotic or handmade paper.

Another concept is to cut up a strip of paper and write the main ideas, a list or a word (using one item  per strip) then glue them in haphazardly on your page.

Write on masking tape, duct tape, electric tape.

Use a label maker to print out words, the modern kind or the old fashioned kind that embosses that enamel type tape.

Fold over a scrap shaped (square, rectangle, circle, triangle) piece of paper an inch (more or less) — the fold over inch is a hinge so paste the back side of the hinge onto the page, add something to the front — word art, picture, date, etc. . . then lift it and write more under it.

Fold a small piece of rectangular paper in thirds — glue down the center back to the page, add your notes onto the paper squares then fold back up.

Cut an envelope in half and glue to the page to create a pocket to add tickets, mini-book of an event, photo, or other small items.
If you use an envelope with a window — to close it up, just paste a piece of paper over the cut edge to give it a folded edge.  Write something on that strip.

Use those page file tabs (separators) on some of your page edges. OR make your own.

Watercolor some paper and then add words, journaling, or glue words from magazines to it.

Use candy wrappers or candy wrapper foil to decorate.  Make sure they are clean and not sticky.

If you use a regular hole punch, save those punches and glue them down here or there.  You will need tweezers to do this then just dip in some glue, and press down on the page.  OR you can use these on a page and put them over any “O” you have written.

Pictures from magazines or off the net — add some of your own conversations in word bubbles.  Scenes from movies can be altered to look like the conversation is about you!

Rubber stamping, or use rubber stamped images.
There are unmounted sheets of stamped words (sometimes called “word salads”) that you can incorporate into your work.

A word about glitter — it can get everywhere for the longest time. Think carefully about whether you want  to use this or not.

Painting pages — ok to do and let dry if you do it in advance but you need to do the testing on the page..  Use wax paper to separate pages if you have to to prevent accidental sticking.  Painting a piece of paper then gluing it onto the page may be a better idea for some types of journals.  If you are painting directly onto the page, you need to let the journal diary stay open long enough to dry.  Sometimes, I do this the night before, let it dry overnight or while I am at work. I paint pages in the future of my diary/jounal not near previous enteries to prevent bleeding.  If painting a page to glue in, you can make a bunch of them at once and use when needed.

On the net — search Doodling, how to doodle, doodling techniques — Flickr has a few pools of various types of doodling art.

Books — Check your scrapbooking store for these titles or order online.

Doodling for Paper Crafters — Leisure Arts 4313
The Doddle Formula — A Step-by-Step Guide — Adrienne Looman
Oodles of Doodles —  Design Originals DO-5305
Doodle it Up —  Natalie Rigdon

Kids Book of Creative Lettering by Lindsay Ostrom (she has written several other books on Creative Lettering)
Lettering in Crazy Cool Quirky Style — A Klutz book

The Journal Junkies Workshop: Visual Ammunition for the Art Addict by Eric M. Scott and David R. Modler — lots of ideas and techniques to make your journal a work of art also.