Thermal Imaging Camera
This is one of those that starts edging into the Expensive Toy category. It's certainly not inexpensive, but I've been surprised at how helpful it has been. It can be quite useful for identifying leaks, and dead cylinders.
Bi-Directional OBD2 Scanner
These days this one should probably be considered a Basic Tool. A quality OBD2 scanner is nearly essential. I have been using this Bluetooth, Bi-Directional Scanner. Wireless is awesome. Only downsides are software can be a bit slow, and it requires a yearly subscription.
Sunroof Drain Cleaning Tool
This is one of those cheap, simple things that transform your life. If you've got a vehicle with a sunroof get one of these! You'll thank me later. A quick cleaning every month or two might keep your sunroof from EVER leaking again!
A/C Gauge Set
Definitely getting more into the Intermediate level work when it comes to A/C but it's really pretty simple. A Gauge Set like this is essential for diagnosing and repairing A/C issues. As I always say, living in FL working A/C is non-negotiable. Save yourself $100's, possibly $1,000's by learning to work on your car's A/C yourself with a gauge set like this.
Find Leaks Fast with a UV Dye Kit
It doesn't matter what you're car is leaking. . .UV Dye can help you find it, and save you a tremendous amount of headache. In fact for some leaks like your AC Refrigerant, you'll probably never be sure you've found the problem without it. Fortunately it's all pretty simple and inexpensive.