A decent guide listing stuff to do with all the components, and explanations of how they all work.
I got the starter kit from oomlout: http://www.oomlout.co.uk/arduino-starter-kit-ardx-p-183.html?zenid=24dc21be8685e2a982387eaae8721293 which was pretty good, but I only have a vague understanding of which resistors to use. Their guide could've done with a bit more theory in it.
At the same time I ordered a laser and I've since got an LED matrix. I think they're both cheap enough to add to your kit.
The servo and motor that came with were good too, so you might want to add those.
The RTC is a good choice. A button battery and socket for it would be useful.
I guess you can't include everything though - I think what you've got is pretty good though you're missing out on a motor and/or servo.
edit: and thow in a couple of header strips, more wire and a bag of mounting nuts and bolts. I want to put something I've made in a box but my local hardware shop doesn't stock bolts small enough to mount the LCD.
You'll be able to pick up bits like that really cheaply in bulk, so should'nt change your costs by much.