The stocks and quests are two of the best ways I've seen. The quests, especially the egg quest, can make you some pretty good tu if you do them daily, and stocks take some watching and getting used to, but once you know them well enough, they're amazing.

I'd just recommend staying away from certain ones, like GLO and MUS. People new to the stocks seem to gravitate toward them, probably because they usually have the lowest prices, but then they never really go any higher and those people give up because they never get anywhere.

Ohyahs. Seasonal eggs are good too, although they take some patience in both finding them and waiting for their price to rise. An egg's price will gradually increase as its holiday gets further away. A Jaaku egg, which is only a million tu now, may be around four million by next May (at a guess), if not more because of the measures being taken this year to cut down on an excess of them.