This is all fairly new to me and by nature very experimental so there are lessons to learn and pitfalls to avoid so post 'em here if you have any that might help others. Some lessons I have learnt so far are.
Don't make your case/housing until you have finished experimenting with your circuit. It's too easy to burn out your project and you'll have wasted time and resources making a case that's redundant.
Set your circuit up temporarily with patch leads and crocodile clips rather than soldering it all up so you can add/remove/change components easily while experimenting. Change one thing at a time and test the effect before attempting the next.
Make notes of what you changed, what values of resitors/components worked best. You will find things by accident and it's easy to forget what you did. If you get a good toy working and you want to make another, it will save a lot of time.
Go careful when drilling holes in the casing for switches, pots etc. Some of them are quite brittle. I tried goinf straight in with a large drill bit. It snagged and tore a lump out of the plastic. Probably should have drilled a small pilot whole first. I'm so used to working in wood with cigar boxes and I was impatient and almost ruined a great toy.