stay west of the 5 freeway if you want anything remotely flat. And San Diego isn't flat. Mostly 30ft/mile is more realistic. Head inland and it's 50-80ft/mile real quick
southbay loop is flat but expect windy w/o any building or structure protection. So it's a trade off.
I'd suggest ride from your hotel North towards Oceanside on the 101. Many places to stop for food /coffee along the way. You will see more of San Diego than just the bay. If you get tired, you can take the train back down from Oceanside to Downtime pending day of week you plan to ride. Not a flat route but beach route.
__________________
Rule #10 // It never gets easier, you just go faster.