Lightwave's Tweaks
This is a compact modification I developed some time ago. It offers several interesting features designed for both adventure maps and regular gameplay.
Main Elements
Glowing Moss Stone

This block creates soft lighting in caves. Its light is insufficient to prevent mob spawning but adequate for navigation in darkness. Generates only in areas with air access. Generation settings can be modified in configuration, including world restrictions.
Dungeon Lantern

Crafted from 4 glowing moss stones and lapis lazuli. Provides slightly more light than moss stone but still allows mob spawning. Perfect for grinders or stylish dungeon decoration.
D'ni Stones

Decorative blocks with numbering system from Myst game series. Emit weak light and can display numbers from 0 to 25. Numbers can be changed through crafting or special tool.
D'ni Chisel

Tool for configuring D'ni stones. Normal use cycles numbers forward, Shift-use cycles backward. Can be applied to blocks or used during crafting.
Glowing Stone Bricks

Stone bricks mixed with glowstone dust. Provide full lighting. In version 0.21, recipe became more affordable - any vanilla stone bricks can be used.
Stone Glass

Durable window made from stone and glass. Also exists in glowing version. Cannot be broken by hand in adventure mode.
Obsidian Glass

Even more durable version of stone glass, inheriting 1/8 of obsidian's blast resistance. Withstands creeper and TNT explosions. Also unbreakable in adventure mode.
Adventure Features
These elements are disabled by default and cannot be crafted.
"Flowers"

Indestructible versions of all vanilla flowers. For decorations that must remain untouched.
Barrier

Analog of barrier block from version 1.8. Invisible unless held nearby. Mod archive contains resource pack that makes them visible.
Invisible Item
Item with transparent texture.
Immortal Rails

Indestructible rails that don't require support blocks. Available in versions: powered, detector, and booster (constantly active powered rails). Became waterproof since version 0.21.