For my mother-in-law's 90th, we had surprise guests. Two of her sons are priests and a daughter is a nun. Mother-in-law is of the old school and pretty much resolved to the idea that her children are cloistered and may not be able to come. But arrangements were made and as we all filed into the chapel for Mass she shrieked with delight when the sons came out of the sacristy to concelebrate for her 90th. A further delight when the daughter went up to do the reading. Maybe you can ask your Mom if there is anyone in particular she would like to see again. Then you can invite this person around as a surprise guest. If there are grandchildren, a little programme of song and dance by them would also be a good idea.