A high-quality reusable vacuum-insulated bottle* costs about as much as 5 lattes at Starbucks, and will last you for decades; you'll probably lose it before it breaks. If you're a typical middle-class American coffee or tea drinker, keeping a portable 16-oz bottle in your day bag will reduce your consumption of disposable paper cups a hundredfold.
Equally importantly, it will improve your drinking experience in three ways: it will keep your hands cool; it will keep your drink hot; it will allow you to carry your drink around without worrying about spillage.
If you're the type of person who brews a pot of coffee in the morning, then, without a vacuum bottle, you're stuck with either lukewarm coffee by midday, or a profligate waste of energy to keep the pot heated. Instead, you can get a large vacuum bottle and fill it from your pot in the morning. It will literally keep the contents piping hot all day long. In fact, I have sometimes filled a bottle in the morning and had warm, drinkable coffee still left over the next morning.
All this may seem kind of underwhelming and obvious, but it is striking how few adults I know carry a thermally insulated bottle. I mean, just go to a coffee shop and look around.
Somehow "thermos" has become associated with childhood lunchboxes and camping. But it shouldn't be. I say this as someone who only recently converted to bottle-carrying, after wasting countless paper cups over the years, frequently cursing when hot coffee splashes up through the hole in the lid as I'm walking. Get a vacuum bottle today! It's awesome!
* a.k.a. "thermos" although that is a semi-genericized brand name, like "kleenex".