A). The one thing I wish I had known before going to Costa Rica: How hard it would be setting up a bank account in CR; it took me seven tries over seven weeks to do so in a credit union; I will open another account in a bank in four months [when I was instructed that they would consider my application. (I attempted to apply both in a nearby provincial capital, tought not conveniently located for me, after I was turned down by the two banks in the town where I live.
Tied for first: Securing affordable traveler's insurance that did not disqualify me on account of age and pre-existing conditions. (I managed to succeed at both and get the bank account established and enroll in traveler's insurance.). I managed to achieve both of these goals, but it took a lot of time and effort to do so.
B). The one thing I wish I had brought with me to Costa Rica: I converted my old IPhone with a SIM card for use in CR, but this action canceled my ability to communicate by text or phone with family and friends in the USA. I made the move with CLARO, which worked best to provide the coverage where I live and travel. Only today did I resolve this situation by buying two (2) inexpensive, new (and basic) flip phones from a carrier in the States and being able to have them "ping" off cell phone towers in the USA at proper intervals to keep them active. Knowing what I know now, I would have attended to that matter before departing for CR.
C). My bags were limited to 50## or a massive overweight fee. So, I jam-packed each of them, and came in just underweight with both. I wish I had brought more books and some personal items of sentimental value, which I have stored in the USA. I may reclaim selectively some of those after massive downsizing in which I cleared out totally my apartment...
I also wish I could have brought more inexpensive (with co-payment insurance) drugs down with me, because the retail price in pharmacies in CR, while less than for comparable uninsured drugs in the US, are relatively pricey as an out-of-pocket expense.
Waiting for a bus can be a hassle, but it's a nice way to interact with the locals and very inexpensive. Cabs are also cheap by US standards. I do not miss the $5,000.00 per year that I expended on my car (insurance, gas, tolls, maintenance and upkeep, registration and inspection fees).
D). I replenished my wardrobe inexpensively upon arrival and could have left half of the clothing behind that I brought down. Anything I needed for home furnishings, I found at Super Walmart, el Rey, Pequeño Mundo, or the local supermarkets such as MegaSuper or AutoMercado, in addition to "mom-and-pop" storefronts in my town.
Because of weight restrictions, My luggage was limited to a rolling duffle bag, a rolling suitcase, a small carry-on rolling suitcase, and a laptop case.
I brought down only a single travel guide. I should have brought other books (English and Spanish), along with such items as a Bible [many of which are accessible online or through apps.
E). I've found plenty of places to buy inexpensive clothing suitable for the Tropics, so I could have left behind half of the clothing that I took and replaced it with other items.
I did take about ten tubes of sunscreen, which is expensive in CR, and hard to get in high SPF ratings.