Sri Lanka doesn’t really have a good infrastructure for budget travelers at this point. There are loads of nice hotels if you are willing to pay US$40 or more per night, but almost nothing below that except for a few hostels. I haven’t been to Palawan but I have been to Boracay and Palawan can’t be too much cheaper. Boracay doesn’t have many budget options either, so my guess is that Thailand or Cambodia is your best bet.
The reason for the recommendation is that Bali is not only a yoga and “spiritual” hot spot, but it’s also the most popular island for solo women travelers in the region. The book and movie “Eat, Pray, Love” was inspired by this trend, and they have in turn inspired thousands of solo female visitors to go there and stay for long periods, particularly in and around the Ubud area. It’s gorgeous and quite cheap once you get there, and the locals are generally gentle and kind to all. Most other tropical hot spots in Asia have far more solo male visitors than solo females. Just avoid lingering in the Kuta Beach area, because it’s packed with bars and loud backpacker party people. Most of the rest of the island is quite mellow.
Since you’ve been to several of the top places in Asia already, and you love it, I think Bali might be your best bet. As I’ve written many times recently, I feel that the main tourist area of Kuta/Legian/Seminyak is just too crowded and overdeveloped to recommend, although it is interesting for a couple days and some nightlife. The arts and crafts town of Ubud is still nice, although also a bit crowded, but it’s not anything like a city so I still think you’d enjoy it.
For our early dinner (Meal 4) we found a burger joint, where I had a bacon and cheese (shredded, not the gross, carb-laden “cheese” sauce) hot dog. I grabbed some green beans with salt and pepper as my side, and sidled up to their topping bar where I smothered my dog with mayo, mustard, onions, and pickles. Dinner was great, and I wasn’t hungry again until much later in the evening (around 9 pm) when the kids and hubby stopped for ice cream. I walked down two shops to the coffee joint and got a plain iced coffee blended with ice, heavy cream, and some sugar free caramel flavor. I accompanied this treat with one packet of my almond butter and was good to go. I sat with my family while they were enjoying their sweet treat and didn’t feel deprived in the least.
Since flights to Mexico City from the US and Canada are usually quite cheap this time of year as well, this could be a perfect short cultural holiday. This city has great shopping, dining, and nightlife, and the central historic and tourist districts are far safer than most people would expect. There are ruins just out of town as well, so a bit of everything is available.