I go to San Diego regularly, will be there next month for a couple of days. I went to the zoo once years ago but it's huge and there is a lot of walking and never been back, it's a one and done. San Diego is surprisingly easy to get around by car most of the time. There is the Gaslamp Quarter to visit, get a picture of Greyfriars Bobby. Old Town is ok, nothing super interesting but worth wondering around and there are plenty good restaurants there too. Don't be fooled by the long lines at Cafe Coyote, it's very popular but I thought the food was so-so. Fred's is better in my opinion. Miguel's has great fish tacos. In May the weather may not be "beach weather" but Mission Beach is worth a look and hanging out at. Little Italy is cool. There are great restaurants all over the place, you can't miss. San Diego Padres may be playing and their tickets are usually quite reasonable but that takes a chunk of time. La Jolla is nice, you can see the seals there but they do stink.