I check online to see what classes are offered and then talk with my advisor to get her approval.

Yes meeting with your advisor is the best idea. See what's available, what's interesting, what is relevant to your major, and what you know you can handle

Make a list of the core requirements that you still need to fulfill, and then, under each core category, make a list of the classes that you are interesting in taking that satisfy that core requirement. See which ones fit into your schedule the best.

Make sure you have back up options, especially if you are a lower classman because the classes that you want may fill up.