Some low fibre foods might be useful a few weeks in to treatment. Crunchy Nut Corn Flakes were about the lowest fibre breakfast cereal I could find, or regular Corn Flakes. Porridge also, but make sure no added bran. Peeled apples, peeled pairs, and bananas are OK, but most other fruit is higher in fibre.
From mid-treatment, incontinence pants can add a great feeling of security, even if you manage not to wet yourself. Not needed by everyone though.
Water bottle is a good idea, at least 1/2 litre, one where you can see the fluid level. When they tell you to drink 6 plastic cups, you can easily lose count around number 4. I used a 1/2 litre Pepsi bottle, and I could drink 2 of those (same volume as 6 cups) without losing count on all except one occasion.
Soft toilet roll, wholesale pack, and maybe wet wipes.