I'll fly in to San Diego to meet my sister for a weekend-long spiritual retreat anywhere within 100 miles. Budget is $500 each.
When you say "spiritual retreat", is this an organized event that you are attending, or are you and your sister in for a quiet weekend "going within"? If it is the latter, then I suggest you both book a place with Airbnb somewhere along the coast and enjoy your weekend on the beach. You can easily get an entire apartment, condo or home for $200 a night for the both of you and most likely for a lot less than that. The Chopra Center is in La Costa Carlsbad and you can't more spiritual than that! In Encinitas, there are Yoga places downtown on Coast Highway. In Cardiff, right above Swami's Beach is the Self-Realization Fellowship. The mediation gardens in the back of the building, off D Street are amazing, with water ponds and a majestic view of the entire San Diego Coast. You cannot get more spiritual than that! They have service there on Sunday as well.
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the above is Self-Realization Fellowship right on the coast in Encinitas. Also, you may want to consider the following:
I have stayed at Deer Park Monastery and enjoyed the daily routine of meditation and quiet time.
Hope this helps.