I've been thinking about the "best" way to earn XP through buying stuff... Here is my thinking: XP is equal to half the amount of money spent on decorations. So if you want to "buy" XP, it would seem wise to find things with the best space-to-xp ratio (since the $$ to XP ratio doesn't change).
For example, the most cost effective single item would be the Rainbow Fountain which costs 5,000,000 but only takes up a space 3X3 squares, giving it a ratio of 555,555 XP/sq.
If you wanted to buy smaller items and get decent XP at the same time, the Luminous Crystals or Sky Trees appear the best at 25,000 XP/sq.
Anybody else thinking about this?
(Of course here I'm not thinking about dragons, habitats, etc... I have no idea what the "best" way to earn XP is if you are not buying stuff.)