If you decided to see Old San Juan on foot it can be done. Tip: If you want to do so but not have it be quite so much walking take a cab from the ship (up hill) to El Morro, and work your way back on foot (down hill).
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If you do go to El Morro, take the cab up and walk back....it's up hill all the way:-)...It the cab ride was $4 each...you can shop all the way back.......it's worth the visit to see it...
In St. Martin an alternative to shopping in Phillipsburg (The Dutch side)- you might consider a shopping tour over to Merigot (The French side)- it is an amazing contrast. There are excursions offered which allow plenty of time to see the island and allow for shopping in both Phillipsburg and Merigot.