Okay! So, I like the idea of them having either vestigial wings or no wings at all, or maybe gliding wings! Give them an incredible leap, climb speed, a confusion breath/aura, and camouflage.
I think they vary in appearance, with some being more chitinous, while others are bark-covered and overgrown with moss, and the elder dragons even having ferns and such growing from their surface bark.
I think one of their attacks is a Vine Snap that deals piercing and poison damage, and grapples the target. They can also Bite, Claw, or use their tail, which is barbed and poisonous.
Legendary Actions would include attacking with any of the above, using camoflauge to disapear, leaping across the battlefield without opportunity attacks, roaring to expand and intensify their aura, and awakening plant creatures or summoning bugs.
Spells, they'd get the control druidic spells, fog cloud, ensnaring strike, and some illusion stuff, like mirror image.
Lair Actions, would be stuff like felling trees to create AoE attacks, making the earth erupt with huge insect plagues from underneath an area, and an at-will sort of effect like speak with animals and plant growth, basically making the land just outright reject the dragon's enemies.
Personality wise, Good Forest Dragons are wise more than smart, curious, enjoy goofs and pranks, and are incredibly vicious defenders of the wild. They tend to enjoy the company of treants, dryads, and other plant races most, but are also known to consort with satyrs and centaurs, firbolgs, and forest gnomes, as well as those wood elves who truly live with nature, rather than simply in the wild.
Evil Forest Dragons are still quite wise, but their wisdom is foul, and turned to seeing into the weaknesses of lesser creatures. They are deceptive, and use their insight and cleverness to corrupt the creatures of the forest, turning healthy woods into the sort of dark forests that villagers speak of in whispers. These dragons enjoy the company of deadly nymps, keplies, hags, giant spiders, corrupt treants, and are often served by blights of all sorts.