My sick day plan is quite simple - eat my normal low carb food if I feel like eating - if I don't feel like eating then I don't eat.
My reason for this is simple - at diagnosis is was told by my DN that I must always eat carbs each no matter how ill I felt. Then I got a really horrendous attack of norovirus, phoned the DN and said I couldn't eat. She told me to force some dry toast down. So I forced myself to eat dry toast between vomiting and urgent trips to the loo and I think eating extended my illness by at least a week because it gave the bug something to live on. The non diabetics around me stopped eating while they had it and it only lasted for 2-3 days whereas I had it for nearly two weeks.