Valley of the Brain Collectors as a sandbox had plenty of optional encounters. Here is one which my players missed.
RED MOULD OGRE SPIDERS 
Huge aberration, unaligned
Armour Class 17 (natural armour)
Hit Points 96 (12d12 +48)
Speed 40 ft., climb 40 ft.
STR 25 (+7) DEX 11 (+0) CON 12 (+1)
INT 2 (-4) WIS 11 (+0) CHA 4 (-3)
Skills Stealth +7
Damage Resistances bludgeoning, piercing, and slashing
Damage Immunities radiation
Condition Immunities mind-affecting effects
Senses blindsight 10ft., darkvision 60ft., passive Perception 10
Languages –
Challenge 6 (2,300)
Mutations. These spiders are currently still suffering 6 points of Constitution drain, and their mutated bodies have lower than normal Dexterity. The same mutations grant a +2 bonus to natural armour, damage reduction, and regeneration. All of these modifications are included in the stats above.
Regeneration. The mutated spider regains 10 hit points at the start of its turn. If the ogre spider takes fire damage, this trait doesn’t function at the start of the spider’s next turn.
Spider Climb. The ogre spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Web Sense. While in contact with a web, the ogre spider knows the exact location of any other creature in contact with the same web.
Web Walker. The ogre spider ignores movement restrictions caused by webbing.
ACTIONS
Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature.
Hit: 14 (1d8 +7) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 9 (2d8) poison damage on a failed save, or half as much damage on a successful one. If the poison damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralysed while poisoned in this way.
Web (Recharge 5-6}. Ranged Weapon Attack: +3 to hit, range 60 ft., one creature.
Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 15 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).
REACTIONS
The cave is filled with red mould. The stuff is radioactive, and any creature in contact with the fungus becomes exposed to low radiation— (DC 10 or 1 level of exhaustion). If disturbed, the mould bursts forth with a cloud of radioactive spores—all within 10 feet of this burst are exposed to medium radiation DC 15 or 2 level of exhaustion).
DESCRIPTION
This towering spider is the size of an elephant. Its legs have spiky joints and its face looks vaguely, but disturbingly, humanoid.