Some Marriott hotels will allow point based (i.e. certificate) or paid upgrades when staying on points. Beach hotels frequently do this. These types of upgrades are not normally suite upgrades but "view" upgrades (i.e. resort view, ocean view, ocean front, balcony, corner, etc.) Some of those upgrades may require more $ or more than one certificate. You can find out more information on them (including ordering them) at Marriott's website: Room Upgrade Information .

As daeus mentions, if the hotel has any "base" level rooms available for redemption, then you can reserve that room and as a Plat hope for a complementary upgrade. The problem here is that everyone is doing that same thing. Thus the base rooms can be quickly reserved leaving only the upgradable rooms as available. If that is the case with you, you might try contacting the hotel's GM and asking about them opening up a base level room for you. Depending on how crowded they anticipate being, the GM may do that for you and even go ahead and upgrade you to a nicer room right then and there. Just remember to not expect that to happen. They may very well be fairly well booked and require you to reserve an upgraded room using extra points or $s.