For these I think...

Users Online, you'd need a checkbox or boolean field in the users table and you could do a query where it prints the names of all users whose boolean is set to online (1) AND not those who are offline (0) if that makes sense.

Last 5 posts,

Just query the place where the posts go to and grab the last 5 assuming you record the dates the posts were made you can do it like that.

Query posts, order by date descending and limit of 5, it will grab the 5 newest posts!