Cartagena, my guess is that it will be cancelled as it usually is. Not really a safe port of call anyway.
If you just want to walk around Aruba, the streets are nice and passable, you should not have a problem with a wheelchair (that is unless they've torn them up since January 2007)
As far as excursions for the Canal. The Coral will go in thru the Gatun locks and come to a stop. Then persons with a ships tour will be taken off onto a tender of some type and taken to shore for the tour. I've done the tour that took you to a tour boat and then you went the rest of the way thru the Canal on a smaller boat, ending up just past the Bridge of the Americas, it is fantastic.
Any of those tours, if wheelchairs cannot be accomodated, shoud indicate such. The crew will do they best to get her off and on the tenders to the tour. If you are still a bit hesitant, just check with them at the tour desk once onboard, but I'd book the tours ahead of time and then if need cancel onboard before the cut off time.
As far as Limon, I'd not wander too far from the secured ships dock area.
