You mentioned that the elective has to be a 2nd/3rd year course, so you have to make sure that the course that you want to take doesn't have prerequisites or that you took it's prerequisites before.
I found this list:
http://www.eng.mcmaster.ca/documents/electives.pdf
I would say:
1. Find a course that interests you from that list.
2. Make sure it doesn't have any prerequisites that you didn't take before.
3. Make sure that it counts as an elective for you... sometimes an elective varies from department to department.
4. Make a thread here and ask people about that course that you want to take...or see if there's a course review for it (here on MI).
Good luck