Avoid getting lost and know when near the ground
Newer players often like to explore, but once they stop exploration, especially in an abandoned mine, they get lost and end up somewhere totally different, away from home. Short of remembering where you last slept and copying the X, Y, and Z values for future editing back to spawn/preferred place, use the following tricks to never get lost:
Explore systematically: If you find a fork in the road, or multiple forks, take them one at a time. When you reach an end, and mining near it yields nothing, then either mark it or use a common block (like gravel) to seal a path off.
Set breadcrumbs: While mining and exploring, be sure to mine up bits of the floor, using cobblestone or a block different from the floor as a pointer to the way out; or an ore block to note an intersection, so you have something to identify where you began later. While this will damage your pick more, knowing the way back is more valuable than a tool, especially since the wooden planks in abandoned mines are abundant.
Keep natural landmarks: Scoop up the first source of water that leads to nowhere, but leave everything else. Some of the water flowing from walls may be useful for exploration. Also, if there is lava out of your way, do not go out of your way to put it out -- it emits light, which may be encased in a structure with glass (panes) as natural lighting.
Know the way outside, and know when there is lava: Torches emit a bright light that fades into warmer tones. Lava has a brighter radius of light, meaning its brightest point is larger. If you see light that you did not create, then it is obviously a lava pool. Outside light on the other hand uses cooler tones. When you are near ground, it does not have the warmth of torch light, taking on a more fluorescent white color near the rock, or at night time, a light shade of blue.
Obsidian is a block that takes 15 to 50 seconds to collect. It requires 15 seconds with a diamond pickaxe, and 50 seconds with anything else. If you are wanting to make a near-indestructible building or structure, obsidian the best choice. Sometimes it is naturally occurring, but often times it must be manually made. First, you will need to find flat lava that is not flowing. Then, collect water with a bucket, and put the water onto the lava. Alternately, find flat water and collect lava. A blackish-purplish block should appear, which is obsidian, and requires a long time to collect. It is much easier to just spawn obsidian, but you will need a mod to be able to spawn it in Single Player mode.
Spleef multiplayer game
Many Minecraft players like to play a free-for-all game called "Spleef". It basically consists of two or more players creating a square stadium (approximately 15 to 20 blocks long or wide) that is floating or above lava or water. The object of the game is to destroy the blocks (stone and dirt are best) around your opponents either with or without tools. Players can choose whichever tools to use, but you can also set rules (for example, no diamond pickaxes). The goal is for the players to try and make their opponents fall through the stadium to eliminate them. The last player standing is the winner.