Defensive Bugs
You will not be able to pass some levels without the assistance of Charlotte's Bug Houses. You can tell if you need the bug houses if your gadgets and plants just can't keep up with how many and how fast the bugs are coming. You need the bug houses to stop their progress so that you'll have time to reposition your gadgets and kill them. Upgrade the houses so you can have multiple defensive bugs out at one time.
Go, Go, Gadget, Go!
You want to upgrade your gadgets first because they are moveable. Get the Super Gnome as fast as possible. This rocket-launching dude really carries a punch. Then get the Super Flamingo next so you can take out those flying creatures. Once you have a couple of your gadgets upgraded, get the quick deploy. Now you're pretty much unstoppable! Once you have improved all of your gadgets, then move on to improving plants and bugs.
