I hear the relationship between units and workload is pretty tenuous, though, so it might be possible to take lots of units without doing as much more work.

The unit-workload correlation is predictable, but not entirely straightforward. In particular:

• IntroSems and other similar freshman/sophomore classes are usually easier than their units would suggest.
• Humanities classes usually have less work per unit than sciences.
• Almost every higher-level math class is 3 units, no matter how much work it is. You can usually expect 5 units worth of work for a 3-unit math class. (This also means that even though a math major takes fewer units than most other majors, it's more work.)
• For people who aren't particularly fast at programming, CS classes can take an extraordinary amount of time (20-30 hours a week for a 5-unit class).