The only new towers you want to buy are Laser for fliers, Flame for creeps, and Beehive which is an all-round useful tower. Tesla and Freeze and Mortars and anything else is a complete waste of time.
Bees from a beehive track to where you are standing, but take a while to get there. If you walk ahead of a target, like the Boss, the bees will all reach him at the right moment and blow him to bits. This was the only way I found to kill the boss in the 4-Trees level.
You can buy the Interest upgrade several times. The first time takes you to 10% interest. The next 15%, and the third 20%, which leads to a lot of money if you're saving. But apart from one of the trophies, this isn't something of concern in normal play.
Can't think of any others at the moment.