Okay, this is my new{to me}favorite idea. I found it in one of my vintage kid craft books and it is so simple - I love it. Clip the lower corners off of envelopes that are destined for the recycling bin and they can be used as bookmarks that simply slip over the corner of the page. {If you seal the envelope the upper corners can be used as well.}
The bookmarks have a bit of room to write a message, like this one{below} - a reminder, perhaps, that I am quickly approaching my Geritol years.
They also work great with magazines, marking out a favorite feature. I like these simple bookmarks for their stealth - I used to use small post-its to mark magazine pages and C would find them and feel compelled to pull every single one out, often ripping pages. These are less likely to attract unwanted attention.
I almost always give books for kid's birthdays and quite often as gifts to adults as well. I think these could be fun to include as a tag when giving a book, perhaps by re-using the corner of a decorative envelope or making a corner out of decorative paper.