QUESTS
My personal favorite way to make easy(ish) tu!
The trick to quests is to stock up beforehand. This can be really daunting, especially for big shops like Fashion Galore, or the Food Quest which uses items from several shops.
Don't worry though, we'll start small.
Visit those shops every 10 minutes and buy anything that's not already in your inventory. If you don't have the item, it will have a white background.
I also like to sort the shop, so that it shows me which items I don't have or what I'm running out of as soon as I visit.
After you buy as much of the shop as you can over a couple hours, you're ready to start the Toy Quest!
TIPS
Don't worry too much if you don't have an item. Click on it to see a handy popover. It'll show you if any are stocked in a Merchant Shop. If it is, you can buy it right there. Isn't that convenient? BUT I wouldn't buy the item if it's over 300k.. maybe 400k if it's a very rare item and I've already made a decent amount of tu from the quest.
Try stalking the Toy/Freaky Toy shop instead and hope it appears within half an hour. If not, it's no big deal to restart the quest.
While you're stocking up for Quests, pay attention to the Merchant price of the item. If it's over 100k in Merchant Shops, it's worth buying extra to stock in your own merchant shop!
See more about that in this post.
The Quests with the best payouts are Toy, Food, Squishy and Clothes. I've listed them in the order I think are the easiest to stock up for.
When starting out or going for the daily rewards, you can definitely do the Weapon and Book quests. They're single shop quests, so they're easy to stock up for, but the payout isn't usually as large as the others.
Tip for the Book Quest - don't bother to buy the books in the Book Shop that are over 1 mill tu. They usually don't show up in the Book Quests anyway.