One of my friends ask me to have a ride with him, and I want to ask what are the places you have been planning to go (assume going to go multiple places) How could I ask informally, what's the most suitable phrase?
This can be covered a number of very simple ways.
There's the popular "Where are we going?" This indicates a tentative agreement to go. Similarly, you might use the simple and direct "Where are you going?" A native speaker, depending on local dialect, might even say "Where all are you/we going?" if multiple destinations are expected.
You can also ask "Where do you plan on going?"
Further, there are a number of even more informal, idiomatic ways of asking the question. You might ask "What's the plan?" or "Where to?" or even something vague like "What's up?" -- anything that connotes a general query about the reason for a trip or the destination. You could even simply ask, "Why?" though that connotes suspicion or a suspicious nature.