Discutir

The main thing to remember about discutir is that it generally suggests some level of conflict or debate in the discussion. As such, it is often closer to argue than to discuss. Here are some examples I have overheard.

• Dejan de discutir. Stop arguing, you two!
• Yo creo que no estamos aquí para discutir. Let's not argue. We're here to have a friendly discussion.
• No me discutas. It's not debatable! There's nothing to discuss. I won't hear of it! You're letting me do this favor for you and that's final. Don't argue. This one was said in a perfectly friendly context, but note that the translation of debate fits. There's an understood conflict, albeit a friendly one.
• Y ya mañana lo discutimos. We'll talk it over tomorrow. The upcoming discussion would have a purpose: to reach a resolution on some matter.

For something general like we discussed his upcoming travel plans, you'd probably want a different verb, such as hablar. However, if the purpose of the discussion were to settle his travel plans, then discutir could fit.

Discusión

Una discusión could be a debate, a heated discussion or an argument. It goes well with the verb entablar, to start or carry out a discussion. Entablar una discusión sobre el tema = have a discussion about the subject. Raise/bring up a subject = tocar un tema. Ahorita no quiero tocar ese tema. I'd prefer not to get into that right now.