- the project
- the pattern (preferably a photo copy of the original, which will be left safe at home)
- a travel tool kit with at least scissors, tape measure, darning needle and optional stitch markers and crochet hook. (I have several such kits that I try to keep with each project)
Ideally, I'd have one of these kits in each car and in my purse. That way I'd always be covered. The hardest part is making myself thing about this when a trip is not immanent. This also requires having multiple sets of needles and other tools. I generally carry my Denise interchangeables, which gives me a lot of flexibility.
My other solution is to take knitting magazines - then you don't have to worry about not having a stitch counter. The only problem with this is resisting the overwhelming temptation to read through them as soon as they're purchased!