I wasted a few too many donuts on low efficiency profit boosting premium decorations before I finally got the Jet Engine Bike unlocked and did the math. White picket fences are the basic profit booster and one of the first you can use. You get 3 free Burning Bush decorations as you buy land expansions (and I got a few from Homer Buddahs) but they are inefficient at only 0.5% extra money and XP for 20 Doughnuts. There are plenty of items that give 2% bonus for 40 Doughnuts, the same as placing 40 White Picket Fences, and take up less room ( even if you double up the fence segments, something I figured out by accident, I do have some double rows of WPF between my Brown Houses) but if you can wait till you unlock the Jet Engine Bike (somewhere in the mid 30's, level wise I think) it give a 2.0% bonus for only 30 Doughnuts!!!
I have a farm of over 100 Jet Engine Bikes now, and I add to it whenever I build up 30 Doughnuts from finding them while visiting neighbors and from the bonus levels (you supposedly can get some randomly from the balloon game in "Sideshow You" at Krustyland, but I've yet to get one after 100+ tries, maybe someday I'll get lucky). I have bought Doughnuts for the game before (several Golden Scratch-R tickets and a few boatloads of Doughnuts) to get all the good premium buildings and characters Wholesale Cigarettes Newport, but now that I'm getting a decent amount from the bonus levels and while visiting neighbors, I doubt I'll buy more, unless they come out with some awesome limited time premium content that is must have.