M.Arch I portfolios will be wide-ranging in terms of their content. Pretty much any creative/visual work is up for inclusion: graphic design, sculpture, painting, drawing, photography, etc. As long as it's your best work, you can't go too wrong. I've even seen people include film projects and writing/articles, depending on what their undergrad backgrounds are. Again, the key is just to put your best foot forward, in terms of overall quality.

Any tips on gaining relevant experience?

You should do a search for other threads on the admissions and application process for M.Arch, as there has been a lot of discussion here on the application process, but here's my 2 cents:

The most important components of your application package will be your portfolio, letters of recommendation, and personal statement

Most people applying to an M.Arch I program will not have "relevant experience" in the working world, but if you have strong portfolio of creative work and can articulate a distinct vision for what you want to pursue w/in the field of architecture, you will be in solid shape. Having good rec letters that speak to your work ethic, intelligence, etc can't hurt either. You also can't go wrong with strong GREs and undergrad GPA.