The one thing that can be problematic in caves is storage. In the UK we had storage space in nearly every room but a cave is different. A cave for renovation will at most only have a small room or two, one purchased already reformed may have built in wardrobes etc but you might decide you need more.
So, to get started you need to plan where and how big the space you are about to create will be. The simple and rustic way is to dig the hole, flick render with sloppy cement to "bind" any loose rock then plaster and paint. If you want doors on the front then obviously carefull measurements will be needed for the frame. If your left with a badly fitting frame go and get your best pal Mr Yeso Rapido. With large spaces such as "walk in wardrobes" and even extra rooms (builders advice to be safe)it is a good idea to arc the ceiling - this will create strength within the ceiling itself kind of like a humpback bridge concept.
I have recently dug out a wine cellar, a room of approximatly 8m2. I placed a thick pine beam above the doorway both for effect and safety, kept the ceiling low to keep more "mass" above and dug out the floor to create height within the room. I cannot stress enough to get builders advice when planning bigger projects.
Smaller decorative things like dug out shelves can be completed from start to finish in 2 or 3 hours, be carefull not go OTT though as a cave with too many shelves all over the walls can look a bit over done! Its easy to get carried away! Decorative touches can include varnished wood on the lip of the shelf, mosaic tiling or natural stone - its basicly down to your own imagination.
One thing to be prepared for is dust and the incredible amount of loose cave rock you will generate. For example, I dug out a hole in our spare room to put in our midi hi-fi. The system is exactly 1.9 cubic feet in size, what came out of the wall was nearly 3 full wheelbarrow fulls of loose rock. When broken up or "dug out" cave rock multiplies in mass as it loses its compact form. Reckon on a ratio of 1:3 As for tools it all depends on your cave rock, my cave rock is relativly soft so I found that hacking away with a pick was fine. Harder cave rock in areas such as Castillejar and some parts of Galera will require an SDS hammer drill to make life easier. Overall it is hard work but one thing you will be amazed at when looking at the size of your cave is that all those years ago the Spanish dug the entire cave by hand, no hammer drills in those days! Its quite incredible.