I'd go for a 4string. I'm in the same boat, not a real bassist but just enjoying playing bass. ?
Try a blues riff and simply speed it up and change the key for the next song.
When in doubt refer to
http://www.bassmasta.net/ for guidance.
It's huge fun. :woohoo:
Wanna get together with me and make a start I'll be glad to help with the basics,
therafter get a real teacher... 8)