how to make an object move - fluently, smoothly - around, in random directions?
You might be looking for 'steering behaviors' (there's plenty of info online on this topic). The 'wander' behavior in particular would probably meet your needs.
That's assuming you're wanting to create some sort of autonomous or AI-driven agent. If it's some other kind of motion you're looking for, you may need to clarify your question a bit.