sorry about the belated response. first up you're in for a treat as it's a great tour, most of things i would cover this forum topic does: http://www.contiki.com/community/discussions/8411-what-do-you-wish-youd-known
other than that i did all the extras but felt like a fair few were either dull or not worth it. the best extra for me was the Monaco dinner as it was probably the best food i had on the tour the worst were two of the Prague extras the black light theatre which was weird and the bunker was dull and most people that did it regreted it because you dont get to much free time so make the most of it.
Since majority of the time is spent on the bus make sure you have a travel pillow and comfy clothes (trackies, flip flops etc.) since whole days can be spent travelling. try not to over pack as you can easily buy things over there and you will be buying souvenirs etc.
make sure to pack some vitamins, painkillers, throat lozenges because of the contiki cough that will be going around you probably wont be able to avoid it but the above stuff should make it less annoying, plus bandaids, antispetic.
it's not much but i hope it helps if you wanted to know anything specific feel free to ask.