You dont mention any specific experience, which makes it a bit difficult to recommend anything.
If you are not experienced with programming/oop/php5, get to learn those first, eg. by making a message board or image gallery.
Otherwise I would say ... How about a simple cms ?
Sound awfully difficult, but its really not...or, it gets as big as you make it.
You got the login/session section at hand and a guestbook for posting text which could be easily modified for other purposes, so why not combine those ? The result could be something like: user management (creation /w email activation), news posting by users, image gallery (with comments ?), administration, etc.
Would be a great way of learning a lot more and hopefully an exercise of a minor project planning (which is a good thing those days).
Just go for it 🙂
