First and foremost if I forget my kit, I'll often just not do things that would generate plastic. Sometimes this means not using/eating/drinking/buying whatever it is, but more often, it's just setting aside the idea of the most convenient method.
Example: If I'm thirsty but don't have my cup, I might choose to get water from a water fountain or coffee in a "for here" cup.
Take away is convenient, but if I don't have the plastic free means to take it with me, I might choose to eat in so I don't have to use disposables.
Eat it All
If I know I don't have a container to bring home my left overs, or compost, I try to think about it before I eat. I'll choose to eat a little less so as not to need a to-go container or doggy bag.
Foil and Paper
When my eyes are still bigger than my stomach and I do end up having left overs, or when I need something at the store, but don't have my bags, I choose paper or aluminum foil instead of plastic or styrofoam. Choosing a more sustainable, recyclable, or compostable option is better than nothing.
Example: If I choose to shop but don't have a reusable bag for bulk items, most grocery shops have a paper bag you can use. If you can't find one in the bakery section, ask someone at check out. Same goes for packing up my groceries for transport. At the diner, I'll ask for just a piece of foil rather than getting the bigger more wasteful option. Foil can be cleaned, reused and recycled, paper bags can be reused or put in the compost.
Choose No Bag
More often than not, I'll just go without a bag if I forget one. We've carried entire grocery orders home in our hands if we forget our bags. You can't do this all the time, but if it's an option, I'll consider it!!
And sometimes, there's nothing you can do, you have to use a plastic cup, fork, portion container, etc... so if all else fails:
Bring it Home
If I have to get a disposable plastic item, even though I tried my hardest not to, I bring it home and reuse or recycle it. I have often been out and been served a plastic fork, despite having brought my own reusable one. I will save that fork, leave it in my bag, car or jacket pocket and reuse it another time. This has come in handy more times than I can count. I still feel bad about having to dispose of it eventually, but at least I know that it got a little more use.
Example: Reuse plastic cups, flatware, etc... Recycle portion cups, and other disposables at home so you know where they end up, compost napkins, wooden chopsticks, and other single use non plastic items, too!