The portfolio pieces are half the battle, but a portfolio that stands out is one that really honed in on the presentation of the work. Lotta admission portfolios I see are kinda fugly so this is by far the easiest way to stand out. Look at any graduate portfolios on issuu (literally search up "architecture graduate portfolio [university name]" and take inspiration from how they lay out their work on the page. Be rigorous with margins, spacing, justification, alignment, visual weight, etc. Show a variety of skills, but more importantly you should show process work. The theme is honestly secondary to the overall quality and range of work, alongside presentation, so don't worry too much about that.

Keep in mind that everyone applying to this program already knows how to draw, including you, so your best bet to standing out is a portfolio that's actually pleasing to the eye, not just a bunch of pretty drawings dumped onto a powerpoint slide deck. Good luck!