Trying to avoid plastic can be a real challenge. In my search for convenient options I found Innate. They make a whole line of stainless steel items. Everything from sippy cups and adult re-usable bottles to a very intriguing line of portable food containers.
I tried the Innate Me-Me cup and we are very pleased with it. The lid is #5 polypropylene. Children can graduate using the A-B-C modular drinking system as they get older so the cup can grow with them and you aren’t stuck buying another cup as soon as they outgrow the sippy spout. The only cons I can think of is the little raised handle on top of the cup. It is a slightly awkward design but nothing that would keep us from using the cup.
I also tried out the Commercial Drive. It is one of the few insulated stainless steel containers that I have found. They suggest you use the lid as a cup but the lid is very small. They refer it to a “shot glass”. You can drink straight out of the base, though that leaves you with an open cup. Using this in the car might be a little unwieldy. But if you want a small container to carry hot drinks or soup, then this would work well. The quality is very good and it feels quite sturdy.