DeAnnaM -Welcome to cruise-chat - the friendliest chatroom on the internet (and the most informative!). The Explorer of the Seas is a megaliner and it would be a great idea to bring a ligthweight wheelchar (called a travel chair) with you to assist your grandmother in getting around and to do things.
Last Fall I brought one with me to assist my girl in getting around both the ship and Bermuda. It was perfect and whenever assistance was needed the ship's crew pitched right in to help! We had a standard balcony, we did not need a handicapped cabin!
Many cruisers prefer cabins in the stern - they are wonderful! It' s the motion of the ocean that does it!