Well not 101, but perhaps a few more than you think.
Starting with the obvious….
• Squawk code + Altitude reporting. On older transponders this was called mods A and C. On a modern transponder, they will be labelled ON and ALT. Always select ALT.
• Emergency codes. We may not expect to use the 7500 hijack code in our little GA aircraft, but 7700 (emergency) and 7600 (radio failure) may be handy. Don’t be shy of using them in those circumstances. They get immediate ATC attention, and pinpoint your aircraft location better than a vague “west of Abberton” which in the heat of the moment may not be entirely accurate.
• Listening squawks. You know the deal (I hope). Each listening squawk goes with a frequency which you monitor – no need to speak. It allows a controller to contact you if they think you are likely to infringe, before the situation becomes serious. And it’s not just “big bad brother watching you”. On one flight home, I was called up “G-EMSA are you on frequency”, “Affirm”, and offered a shortcut through the Stansted zone. Very helpful.
• 7010. This is NOT a generic circuit squawk. It should only be used at airfields that instruct you to use it – they will have an agreement to that effect with ATC. So Audley End, Andrewsfield and North Weald all use it because of their proximity to Stansted.
And a few less obvious….
• Flight timer. Most modern transponders start recording flight time immediately upon take-off. You can use this to record times at waypoints in your PLOG, or to back0caluclate take-off time if you forget to note it down.
• Up/down timers. A digital stopwatch is just a few button pushes away. Handy for detours and for timing instrument approaches, holds and the like. The transponder is more or less in eye-line, unlike a stopwatch on a kneeboard which requires you to look down.
• Lost 0030. If you are lost (or euphemistically “temporarily uncertain of position”), then 0030 is the code to use. It requires you to speak to someone, but also alerts ATC of someone needing help or who might infringe. Who might you talk to – the relevant Radar unit or LARS controller, but failing that D&D (Distress and Diversion, not Dungeons and Dragons) on 121.5 is a good bet.