If you're playing with arpeggios you're going to be playing the notes in a chord. Pretty much all standard chords can be found here:
If you've got a piano or keyboard (virtual or not), play around with them and find something you like. Usually an arp function on a synth will play the notes in sequence traveling up, down, up and back down (with and without a repeat of the top and bottom notes), and random.
I almost wrote that giving an instrument just a few chords or just one key seriously diminishes its usefulness, but then I remembered you're building this thing for $5 and probably just for fun. Just find some notes that sound pretty together and go nuts. Post a vid when its done.
As for my progress, I've not made any. Between work and study the Auduino has been on hold, and will probably remain so until after the holiday (I'm in America). The damn thing still keeps me up at night thinking of solutions or features, so I'm still excited about it. I hope to have a fully working prototype soon. As it turns out I'm short by one analog input, so I think I'll get myself a Sparkfun Pro Mini 328 for the final product.
https://www.sparkfun.com/products/11113?
This bad boy is smaller, and has 8 analog ins. Seems perfect for the final product. I'm also considering designs for the case. I think I'll be designing something to be built through Shapeways, since they're rather affordable and I'll be able to get exactly what I want.