Will this be indoors all year or outdoors during the summer? There are any number of tropicals and temperate plants that could make good bonsai -- take a look at some online bonsai stores that sell cheap "pre-bonsai" and get a couple of varieties to test and practice training techniques on.

My limited experience sez avoid the ever-popular serissa foetida, because it's likely to die quickly if you miss a watering: I'd try malpighia species, vining jasminums or trachelospermums because they're tough and grow fast, Pithecellobuim flexicaule (Texas ebony), or murreya paniculata (orange jessamine).

If you want to stick with succulent bonsai, there's adeniums, trichodiademas, flowering crassulas, begonia partita . . .

Have fun!

Jim