If you’re looking for a more reasonably priced Mother’s Day buffet brunch (although they may all be booked up):
Bahia Resort on Mission Bay (we’ve been to a few of their holiday brunches)
Catamaran Resort on Mission Bay (sister resort; both of these brunches include a ride on their sternwheelers)
Tom Ham’s Lighthouse (great views of San Diego Bay and downtown)
Wow, I cannot imagine eating $189 worth of brunch per person! That’s way too rich for our blood! We will be in San Diego later this month as well. Will avoid that brunch.
Even if you do not go to the Del for brunch, I would recommend a walk through the property. I have been lucky enough to stay at the Del on business trips for DH over the past 20 years or so and what I really like about the Del is that you don’t have to eat there. It is easy to walk out from the hotel into Coronado. We have nice dinners there although no particular place immediately comes to mind. There is a casual seafood place across the Del and up a bit on the left heading down toward the state park. I enjoyed the tour of Midway when we were last there. Also Balboa Park is quite lovely. Torrey Pines and a drive through La Jolla itself very enjoyable. The zoo is of course great but takes a good amount of time to do.
FYI, the Mother’s Day brunch at The Del is $130-140 for adults but by the time you add in taxes and gratuity, yeah, it’ll get up there. The regular Sunday brunch cost is $98. Definitely still not cheap but my guess is that’ll be the only meal you need that day. 
There are other dining options available at the hotel. There are some spots where you can grab a lunch outside enjoying the setting and the views for less money than Sunday brunch.
@doschicos you’re right -I think the $189 was for the Westgate Hotel. But $140 is still too pricey, unfortunately.
I don’t know if Hash House takes reservations. It didn’t a few years ago, when I went. Being the parent of a server didn’t get us in quicker but our daughter’s advice to arrive early did. The portions were huge and didn’t cost $189.
Yes, the brunch prices are for the holiday and probably the highest of the year. If you head down Orange Avenue from the Del, there’s a great ice cream parlor, Moo Time. Go there rather than the outpost in the Del. Also inside the Del off the lobby is a gift shop filled with photos and memorabilia from the history and movies, kind of a mini museum. If you’re in a car, drive down Silver Strand Blvd. just a little ways (it’s an isthmus) and you can see some the Navy Seals training areas on the beaches.
Your husband and father will really enjoy the food options inside Petco Park. Many of the local eateries and breweries are represented there.
In Balboa Park, there’s a charming shopping area called Spanish Village Art Center filled with local artisan shops. At the far end is a great stone and gem shop with wonderful original jewelry.
C-level on Harbor Island, near the airport. Casual outdoor dining, no reservations, amazing view.
I’ll second Duke’s in La Jolla. Its not distinctly San Diegan, but the food is great and atmosphere re very good.
My recommendations:
Breakfast
Beach Grass Cafe in Solana Beach. I especially like their unique blue corn pancakes. http://www.beachgrasscafe.com/
Lunch or Dinner
Lorna’s Kitchen. a moderately-priced family restaurant that serves far better Italian food (IMO) than the pricier places in Little Italy. I recommend the Tagliatelle Bolognese.
http://www.lornasitalian.com/
If you drive out to Julian, on the same trip you can drive up Palomar Mountain to the observatory and see the famous Hale telescope with its 200 inch mirror. They also have a nice astronomy museum there. Plus the mountain itself is nice. You are out of the scrub chaparral that typifies most of San Diego and are up in the piney woods, There are some nice hiking trails in the area too.
San Diego is the oldest city in California, so if you like history you can tour the missions which were built in the 1700s…Mission San Diego de Alcala and Mission San Luis Rey de Francia in Oceanside are both very nicely restored.
Balboa Park Gardens are nice-most are free. My favorite is the Desert Garden for the unusual cacti.
Other breakfast places we like include Breakfast Republic and Snooze.
Italian - Buona Forchetta in North Park
Extraordinary Desserts for the beautiful desserts decorated with flowers
Other Tacos places not mentioned above: Trujillo’s by SDSU; Carnitas Snack Shack; Lucha Libra are a few we have tried
Hash House and Snooze both get crowded so go early. I like Hash House much better than Snooze. Breakfast Republic is good also and there is one in Liberty Station. Buona Forchetta is excellent, but the one in South Park is really busy so I recommend going to the one in Liberty Station.
On your way to or from Cabrillo National Monument, stop at the cemetery coralbrook mentioned, Fort Rosecrans. National Cemetary. https://www.cem.va.gov/cems/nchp/ftrosecrans.asp It’s a federal cemetary like Arlington. iIf your parents have the US Park Service Senior Pass, it gets you into Cabrillo for free so bring it along. I bought one the first weekend we lived in San Diego when they were still $10.
Yes as noted above do not miss Extraordinary Desserts. My son stumbled upon it when we were walking back to our hotel from a terrific dinner in Little Italy. I am not all that into dessert but even I could not help being impressed by the variety and quality of their offerings. My son was in heaven and I sampled everything he ordered which was quite a lot. They also serve some nice wines by the glass. It’s a lovely way to end an evening.
It will be crowded so expect a wait.
If you like craft beers, you must try the local offerings such as Stone, Pizza Port, Alesmith, Green Flash, etc Many of the breweries have a restaurant associated with them with pretty good food. La Jolla Cove is fun to check out if you can find a parking spot. Great views while hiking at Torrey Pines.