First I checked Amazon, and found a decent plastic tray for about $17 (LINK). That was okay, but it's just plastic and is bound to take some preschooler abuse, so $17 seemed like maybe just a tiny bit more than I wanted to spend for something that would likely end up being almost disposable.
We were in Hobby Lobby shortly after that initial internet search, and found a Multi-Purpose Lap Tray (LINK) for $6.99. As far as I can tell, these seem to be fairly identical. It was in the section next to the kids' crafts and toys - so while it would definitely work for an adult, I think it was geared for children. Perhaps most importantly to Pickle, it came in pink. Seeing as how she told me just last night that she "just wants everything around to be pink," this was a pretty big bonus.
This set up is really convenient. There's the little tabletop part on the top which is the drawing/crafting surface. Off to the one side, there's a deep pocket that goes the entire length of the table - this is great for spare construction paper, notebooks, anything you might need. On the other side of the flat surface, there's a cup holder and a separate little pocket. We store markers, kiddy-safe scissors, glue sticks, and so on in that smaller pocket. The cup holder is perfect for a sippy cup, but we also have put a glass of water in there so that she can use watercolors before. It's really convenient and kids love to hide crap all over the place, so it also makes Pickle feel like she has her own little special cubbies that are just hers. It was definitely worth the $7 we paid for it.
I have read that some people use this tray in bed or in the car (or a child uses it in the carseat). It's not incredibly durable, so I'm not sure how well it would work for that - Pickle put a crack in one edge of it by putting a lot of her weight on it (keeping in mind that she weighs less than 40 lbs). We'll likely be replacing this one in the near future, but it's continued to be usable even with that cracked edge.
The only other minute complaint I might have about this would just be that there is a lip around the edge of the flat area. If, let's say hypothetically, glitter happens to spill into that little area, it's almost impossible to clean out. Pickle does occasionally eat snacks at her little tray, as well, so it's also a food trap. I've dug dried paint and such out, so it's definitely possible, but we have since learned to just put down wax paper before doing any crafts where much of anything can spill.
Pickle loves her little table, and it saves our floor and furniture. All in all, a pretty solid win for $7.