Yes, credit cards are all you need. Not having an open loan present (and almost paid off) will keep your scores about 30 points less, but those 30 points aren't absolutely necessary. You'll be fine with just credit cards and time. Just be sure to always pay them on time every month and keep your utilization low. Over time, your scores can reach 800 with just a few credit cards active even without an installment loan open/present.
