San Diego area roadtrips & family outings

[Sub-Urbanite]

I'll have the rental car while the family is at Legoland. I assume I'll have at least six hours of freedom and I plan on going east to Anza-Borrego for a hike.

For the return drive, how much consideration can I give to going east to CA 86 and returning to San Diego on I-8 if I'm doing this on a weekday?

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On the other side of the border, I want to take my 2nd-grade daughter to Mexico for a day. I haven't been to Tijuana in 25 years. Thoughts on walking over the border and using that as a cultural experience, vs. renting a car there & driving to Ensenada, vs. driving to Tecate and just walking around there?