Here's something to aspire to - being able to wring miracles out of the ATTiny as well as this guy: Electronic Lives Manufacturing.
Using just an ATTiny45 and a loudspeaker, he's able to produce a full, polyphonic musical box using wavetable synthesis, capable of playing tunes with over 300 notes. It's a far cry from those musical greetings cards inserts, which are usually just monophonic square wave tone generators and a piezo transducer.
To achieve the necessary speed and compactness (just 4KB of flash space), it's all written in AVR assembler. I'm not sure I've got time, or room in my head, to learn yet another assembler instruction set, but this is certainly some pretty serious inspiration to get the most of these amazing little chips.
Given that I've got ATTiny85s, with double the flash space, I'd quite like to tinker and add some simple extra functionality to this (e.g. deep sleep mode, playing one cycle of the tune for each press of the reset button; and my own choice of tune) but that might have to wait a while.