Dragon Effects

What are the lair effects of a red dragon, and how can they manifest in interesting ways?

Lair Actions (occur on init count 20) - can't use the same action twice in a row:
-magma erupts from the ground within 120' creating 20' high geyser with a 5' radius. DC15 Dex ST, 6d6 fire damage or half
-tremor shakes the lair in 60' radius, DC15 Dex ST or knocked prone
-volcanic gases form in 20' radius sphere within 120', lasts until next round, DC13 Con ST or poisoned until the end of its turn, incapacitated

Regional Effects - one or more of the following (fade 1d10 days after dragon's death)
-small earthquakes are common within 6 miles of the lair
-water sources within 1 mile of the lair are supernaturally warm and tainted by sulphur
-rocky fissures within 1 mile of the lair form portals to the Elemental Plane of Fire, allowing elemental fire creatures to enter

Ok, I seem to be developing the end game for this adventure path from the end back to where the party is. That is fine. Frankly, I think it is the best way to design any encounter or run of encounters where you expect them to play critical roles in the adventure. This is simple to explain, the essence is to create the monster, give them a home and a habitat, a reason to be where they are, then explain how they'll defend themselves. It creates a believable scenario, believable intelligent acting adversaries, and a playable defense for an encounter that matters.

The lair actions and regional effects are essential for properly playing the area approaching the lair. I've just pulled the info straight from the MM. I think I will tweak the regional effects to expand their impact. The following is a detail of the area in question.

I've left enough to the east to show the location of the swamp and the direction of the Slaad village. In fact, the party will probably pick up their journey at the red dot on the right, 100 miles from the volcano's peak.
The white cross is the actual peak of the volcano. The entrance to the dragon's lair is on the southwestern slope, with the entrance to the kobold lair on the northeastern slope. I've added trails that are likely in play (note that the Dragon Cult has no direct connection to the volcano or the kobolds. They are at the fringe and slightly separate, though I have decided how or why yet. I feel there should be something at the center red dot, fifty miles from the swamp to the peak. My thought is for the party to start toward the peak, divert to the cult, then return to the kobold lair to enter the mountain.
The kobolds have a warren of tunnels and dens, with a series of natural caves and tunnels through the heart of the volcano leading to the dragon's lair. I think any part of the mountain, at least under it, will be subject to the regional effects of the lair. The kobolds sit at the outer limit of this effect, and it is part of the reason approach through their lair into the dragon lair can be made without alerting the dragon directly.
I need to map the cult fortification complex, the kobold lair, the volcano, the dragon's lair, and create an encounter for the fifty mile mark. At least the game is closing in on completion. I think I need to stat out the actual red dragon, and the leaders of the cult. The rest of the areas can be stocked with more generic entries, although the fifty mile encounter should feature something specific rather than general.