Suggestions for notifying user on changes?

I'm working on an app for collectively writing stories and I have most
of the basic functions done. Users can create Universes that act as
containers for Characters, Locations, and Events. Characters have
Experiences of those Events. Universes start out completely private,
but the creator can grant three levels of access to other users. I
keep track of who created a Universe, who create a resource within
that universe, and who last made changes to that resource.

I would like to have some way to notify users of what has changed
since they last logged in. What records have changed, etc. I thought
about going the email route, but I don't think I want to do that since
many small changes could cause many emails to be sent for the same
thing, though eventually this might be the best choice. I'm thinking
about maybe making a model that belongs to :user, that keeps track of
changes, and using Observers to update the table. But I'm not really
sure of the best way to do this. I could probably set up a cron job to
crawl that table once a day and send emails to each user, that way I
don't have multiple emails per user per day. I wonder what systems
have other people used and what pitfalls have you run into?

The first paragraph is a great idea!

The second is terrible! Using a cron job is almost as silly as emailing
(but not quite, at least you will be keeping your silliness to

Why don't you just collect all this information when they next log in?
If you have timestamps for all events, including logging in and out, you
just compile a little "digest" of events that have occurred since the
last log in, and then the user is free to examine that at their leisure.

What I meant by the second is the same way digests are sent out here.
Rather than an email per incident, you'd get at most one per day that
lists changes since the prior day.

That is true, I do have a created at and updated at time for each
thing, so I could highlight those that are new since the last logon.
But I'd have to keep track of what they have seen and what they
haven't, since if they go into one Universe and fiddle around for a
bit they may not have actually seen everything there by the time they
log out, nor seen anything in the other Universes they have access to
or have created.