From Cozumel the only impressive ruins require a ferry ride to the mainland. The ferry is not as smooth as travel on your cruise ship, so that might make a difference to you. The ruins at Tulum are indeed spectacular. I've been there twice and really enjoyed both visits. I would go again. The ruins at Coba (also on the mainland) are more impressive in their size and scope, but it is harder to get to, especially given the ferry time. I don't believe the cruise lines offer many tours to Coba.
In Costa Maya the ruins at Chacchoben are an easy tour and quite impressive with two large pyramid temples and many other smaller buildings and walls. There are other ruins that are probably more impressive (Kohunlich) but are LONG ride from the cruise terminal.
In Belize there are fascinating ruins at Altun Ha (as previously mentioned), also Xunantunich and Lamanai. Either of the three will be an interesting look into the past.
I love visiting the Mayan sites on our cruises. I have never been disappointed, but each ruin offers something different.
2004 - Fascination
2005 - Ecstasy, Elation
2006 - Sensation, Holiday
2007 - Fantasy, Glory
2008 - Holiday, Sensation
2009 - Fantasy, Valor, Glory
2010 - Monarch OTS