I've already pulled apart the Chrissy lights and they'll be dead simple to work with - well apart from the points where I have to drill holes in the console for them. You can also give them an interesting look by inserting some into things like ping pong balls, which diffuse the light and look a bit different.
But things are getting interesting. While researching LEDs and how to work with them (since I starting at a point where literally know nothing about electronics) I have found some quite nifty and cheap projects.
A really, really simple one is three RGB (Reg Green Blue) LEDs hooked up to a battery, popped behind a bit of semi-transparent plastic. Without anything controlling them, they change colour at different rates, creating different patterns and looks. That would cost less than a dollar.
The total cost of parts for couple of photo transistors and other parts that would light up some LEDs briefly when you waved your hand over them is a dollar or two.
Those are just two examples. Of course the learning curve here is going to be huge, but the more I look into it, the more surprisingly easy it would be to do. It helps that I don't have a time limit, and if I find the more interesting stuff is beyond me, I'd be quite happy to just have the Chrissy lights on their own. TLex and GodZoe won't care. If it has a bunch of buttons and switches to play with, and some flashing lights, that'll be plenty for them to have an absolute ball.
Oh, and I suddenly realised/remembered tonight that people hack the sound chips that you get in some greetings cards. If I can learn to do that, I can create cheap audio interactivity.
I like the fun I'm having just discovering this stuff, and learning new skills is always good. If I get this finished before GodZoe goes to school, I'll be happy. Worst case, it'll be the weirdest 21st birthday present she's likely to get.