Cbeebies have podcasts you can download to add to your DVD, apps etc stash. Stories and jokes etc as well as music and fun.
Having a dvd player, especially if there are earphones attached, is GREAT!! iPods can be great too if DCs are techie.
I spy, counting numbers of specific colours of cars, guessing how far a mile is,...and any other number of general "car" games are vital to have in the memory banks (or listed in your handbag notebook).
"Stop the bus" can work well too (someone starts at a and goes through alphabet silently in their head til someone else shouts "STB", that gives you the letter. Then everyone has to think of a boy's name, girl's name, animal, place, and food (could add sport, celebrity etc based on numbers and abilities) beginning with that letter - 1st to think of all shouts STB and gets 5 points. Then everyone says what they thought of (so all boy's names first, then all girls names etc) and for everyone that a person gets, 5 points. Mark down totals. Start again with next person choosing letter).
Sing-songs - practise your carols for Granny etc.
And then make sure that there are quiet activities too.
Colouring pages (use crayons or twistables rather than having to pare pencils or deal with felt tip stains), activity sheets, stickers etc are all good to have on hand too. Reading should be limited (see next point) but can be allowed for a while if not prone to motion sickness.
If kids aren't terribly used to long journeys or prone to illness at ALL, pack a spare outfit within easy reach. I tend to keep a tracksuit, facecloth, tissues and wipes in a plastic bag - just in case.
Snacks that are not messy, and a mix of treats and nutrition, are good. So some jellies but some dried or fresh fruit and plain/fruit biscuits too, cold meat/sambos/quiche are good picnic items. Chocolate can be messy, crisps can be smelly, but that's up to you to decide your boundareies. Drinks in sippy cups, travel cups, sports bottles, tetra packs with straws if need be etc. A flask of hot water to make tea/coffee/hot choc/cupa soups is very helpful (with sachets in a ziploc bag), even a flask of hot milk if milky coffee and hot choc or even plain hot milk are preferred - warm milky drinks can be good in evenings if long way yet to go. (You may not need these, but may not pass services at the right time, or may have delays).
Wipes, tissues, lip balm, travel aromatherapy oils (eg. Neal's Yard roll on for wrists for travel, stress, sleep etc) are all handy to have within reach at all times. Keep some paracetemol and kid's pain relief to hand as well, and perhaps some rescue remedy if that helps you or them.
Bring small blankets/rugs and cushions for heads to allow sleep. You could use coats instead of rugs, but they can be awkward (I use an old cot blanket of DD's so relatively small volume). Also bring a favourite snuggly toy for when tired.
Use maps. Print off 2 copies of the route from AA/RAC/Google maps etc. 1 of these are for the driver/navigator. The second is for the back seat and can be used to show progress, but also look at where you are now, talk about landmarks en route, etc. (Lots of different learning opportunities if you wanted!! 
) Even if you have a satnav, have at least a map of the expected route as above but preferably a proper map of the entire journey so that if diversions happen and batteries die, you can still move onwards or find stopping places.
Can presents be posted ahead? Or at least some of them? Or ordered online and delivered to the destination rather than your home?