We have a couple of small storage boxes, ~20x10cm ish, which are not hard plastic but kind of squishable. I got them from home bargains or similar. I use these for them to play Duplo, Lego, play dough, magnatiles etc, as they can play with it and have somewhere to put bits down. It's also ideal for passing things into the back seats and catching sick.
I use drawstring bags to store and organise small amounts of the above, Lego, Duplo, magnatiles etc, or other toys in groups ie a few animals and a farmer, two tubs of play dough and a couple of play dough toys, etc.
Toy/activity ideas in addition to those mentioned
- toilet roll tubes and stickers
- sticker books
- magnatiles (or playmags?) do mini travel ones which look good
- a purse filled with old cards/ loyalty cards
- yoto mini
- small ish books
- random fidget toy type things
- Wikistixx
The above is admittedly mostly useful for the 3yo. But when ours were babies we used to time it all around their naps, so assuming two naps a day then trying to drive as far as possible during morning nap without it being a monster nap, then do a long lunch somewhere like a national trust site where we'd be there for a few hours, then longest drive after that when the baby would ideally nap for like 3 hours 🙏🏼
Experience - have driven from Somerset to northern Scotland on more than one occasion 😆 done over two days but both days would be 5-6 hours driving time.