Look at dogs and cats for inspiration. Besides doing things with their tongues (as CaptainSkyfish suggested; good suggestion!), think also about angle of their head. Do they have mobile ears (or ear-like fins)? If so, these will almost certainly be expressive. It's not much of a stretch to give them moderately expressive lip muscles, that will allow them to raise or lower the corners of their mouths (for a smile or frown), raise their lip over a single canine (fang) vs. their whole mouth (a sneer vs. a smile/grin), or make an 'o' at the tip of their snout (surprise). Even whether or not they pull their lips back, which many animals can already do, can be the difference between aggression and happiness.
Realistically, though, you should also be thinking about whole body expressions. A lashing tail can mean anger (cats) or happiness (dogs). A raised tail can mean contentment. A tucked tail can mean fear / submissiveness. An arched back can mean a number of things. A low sternum with raised hindquarters can mean playfulness or... er, 'mature playfulness'. If you look to existing animals for inspiration, you will find there is plenty of room for expressiveness.
Also, don't be afraid to make your dragons hard for humans to read! There is a time and place for making your characters relatable, but there is also a time and place for making them alien!
p.s. Another question just posted this, which may give you some inspiration.