For many simple languages, the most complex construct is the expression. Lots of things come to light there. Lots of recursion/fun with stacks, operator preced

For many simple languages, the most complex construct is the expression. Lots of things come to light there. Lots of recursion/fun with stacks, operator precedence, the type system, parameter passing. Pretty much a good solid chunk of language is wrapped up in expressions. Get expressions working, and the rest starts to readily fall into place.