Re: Some coding questions.
sitewide activity table is just cache of all the activity for every user. It would crash your server if it wasn’t a cache. Every user has a table where their activity is not cached, you’d be better off using that.
I’m not sure exactly where you are trying to use the $activity object? The activity tables have to be generic, so that they can be used for anything, adding specific fields wouldn’t work. You need to check out the format_activity function in each component, that is where you would want to replace the output with the excerpt.
Your best bet is to download the skeleton component, as that explains how things work: