Apart from knowing a bit about aperture and exposure and so forth, make sure you know how to use your camera. Can you change exposure without taking your eye off the viewfinder? Do you know how to dial in exposure compensation. Do you understand all the different metering or autofocus options on your camera, what situations to use them, and how to quickly change them? If not, pick one thing per week, read the manual, then experiment. Only when the technical stuff is second nature will you be able to forget all about that and be creative without being distracted by fumbling with menus and settings.
To work on creative side of things, and push those boundaries of your technical knowledge, I'd suggest:
Besides rule of thirds, research other compositional elements, and go out and take shots which exploit things like
- negative space
- color contrast
- leading lines
- patterns and texture
Try different types/styles of photography - each will give you new challenges and things to learn:
- do some portraits - work on lighting, post processing perhaps
- landscapes - work on composition, DOF
- abstract - composition, shapes, textures
- try some black and white conversions - shoot some images with B&W in mind
- street photography
- product or still life - bowl of fruit or some jewelry
- motion - sports or birds in flight, work on focus, panning technique
- night photography - handheld multi-second exposures
- golden hour - sunrise/sunset
- get in close, very close if you have a macro lens
- experiment with focus, DOF
Carry your camera with you every day. Every day take at least a few shots of something you see. Can be an ordinary thing, but try not to take an ordinary snapshot - try an interesting perspective or angle, convert it to B&W, do something to make it special.
Go some place like your kitchen or garden. Take 50 photos. Experiment with focus, perspective, exposure. Review these and see what you like. Do it again, same place. Another 50. Keep at it, you'll start to unlock some creativity.
As mattdm said, as you go, keep a portfolio of your best shots. I don't consider myself that great, but I've been doing this as a hobby for 30 years, and what pleases me is my best shots are always some of my most recent ones, so I know I'm still improving.