I don't think that Tijuana is good for more than a day trip from Sand Diego or 2 days tops.
Mexico is a large country.
Cancun is more popular with younger crowds - clubs, beach, water sports, underage dringking, etc. There is also Tulum nearby (ruins).
Isla mujeres, off the cancun cost coast, is a nice place.
Cazumel: close to Cancun
Veracruz, less crowded than Cancun more "real".
Mexico City is instersting - 24 Million people live there! Museums, parks, ruins, hanging gardens, etc. People everywhere!
In the pacific:
Cabo San Lucas (Baja)
I would pick any of the above, except Tijuana