Workaround:
- create these activities in a section "on the bottom" of a course
- then one-by-one:
- create hyperlinks to these activities (right mouse menu)
- paste these link on the book-pages
We have implemented this at the OU, but we have not contributed it back to the community because we thought it would be controversial, and that no one else would want it.
We call it 'Stealth mode'. That is, in addition to the standard 'Visible' and 'Hidden' an activity can be 'Stealthed'.
I think that is a great idea. I want to make the participants go through the material that is set up in a book to get the forums and assignments. Right now several went straight to the forums and assignments and tried to skip reading the required material then of course they were confused because it didn't make sense to them.
Thanks for your comments they were very helpful!!
My students (being school age) need to have a limited amount of info on the page at a time.
A book is a great way of helping achieve this...but it has to have interactive elements inside it or I might as well give them a printed copy!
Interested to know why you thought it might be controversial?
The ability to make resources and activities unseen without making them unavailable is a frequently requested feature. People have suggested some interesting workarounds, but none of them is perfect. For example, here are two recent discussions I have indexed:
http://moodle.org/mod/forum/discuss.php?d=62201
http://moodle.org/mod/forum/discuss.php?d=72887
In the second of these, Sam Marshall volunteered to contribute the feature to core Moodle.
Also see MDL-4782 "allow resources/activities to be hidden but available/active" (22 votes).
This is exactly what I too wanted last week. I did a "workround" but it wasn't entirely hidden but it did suit my needs slightly better than the ones suggested here (apart from Tim's). I effectively removed the activity/resource text link from the course page/activity block - unfortunately the icon remained 
, but I could link to it from wherever I wanted and users could only access it from these points, e.g. at a point in a Book. Here's how I did it.
Create the "thing" (technical term!) you want to link to. Open it and copy the url onto your clipboard. Go back into the "thing" giving it the alternative name of...
</a> <a>
...and save it. Use the url you have on the clipboard to create the link, et voila ! A link to the (semi)invisible "thing". I thought I was being dead clever!!!
- obviously not clever enough as it didn't remove the icon like Tim's did!
It would be really good to actually use this "stealth" mode. I'm sure that others could make creative use of such a feature however controversial (he types while cheekily whistling "Please release it, let it go......." 
)
Carl
There is a block that you can get from the Modules and plugins page called a side bar (here is the link). It's like a course page - it has drop down lists so you can create resources/activities - but within a block. You hide it in the usual way by using the hide/unhide eye icon.
Usually when you hide objects on a course page they become unavailable, but this seems to keep them available, you just can't see where they are. If I've found a bug, it doesn't need fixing!!! I like it just the way it is!!
Carl