Tools – take a few extra shovels and picks (note, an iron or better pickaxe is required for many ores). An extra sword might be worth bringing too, while a bow may be unnecessary.
Torches and food – obviously, take more. Caves are dark and mining is tiring. Although, soon enough you should mine enough coal to self-sustain, in case that you need extra torches, so long as you took planks.
Clock – optional, but will show you the current time up on the surface. Before you choose to barge out into a mob-infested night above ground. A bed isn’t necessary below ground.
Buckets – useful for gathering lava, breaking falls with water, or helping you clear the way, including turning lava into obsidian (hence, its worth taking a bucket pre-filled with water).
Part of my concise inventory series: https://premiumminecraft.com/survival-inventory-layout-and-space-management/