If you live in the U.S., or a few other select countries, you can earn 15 nights per year with a personal Marriott BONV°Y™ credit card, and another 15 nights with a business Marriott BONV°Y™ AmEx card ($125/yr., w/ 35K-point certificate).

This will get you 30 nights annually toward the 50-night requirement. There is a $95 Chase Visa that gives a 35K-point cert., or a no-annual-fee card without.

If you want to splurge on the $650 Marriott BONV°Y™ Brilliant AmEx card as your personal card, you would earn 25 instead of 15 nights for that, bringing your personal plus business card total to 40 for the year (25 + 15), leaving only 10 nights to stay in a hotel to attain Platinum Elite status for the year, one or two of which can be with the annual certificate(s) and receiving Platinum Elite status as a card benefit.