So exciting!
I have heard that Riu is nice. I had a friend stay there last year and she said they had a great time!
I'm not sure about the sand and candle center pieces, but I do know that if you want flowers included in them, it will most likely be extra (you only have 1 centerpiece included). I am doing pretty basic center pieces that only incorporate a couple flowers in each, and ours were extra (but we are only going to have 5 or 6 tables, so since we don't need that many, I was okay with the extra charge). The table runners are included, and I believe any tea candles that you want to have on the table are included. I would still just double check with Lizbeth.
I had a million ideas too over the course of our wedding planning process! It really helped me to make a bunch of Pinterest boards to collect my ideas, then when it came time to really start planning, I sifted through them all and made a little PowerPoint presentation of the stuff that I really wanted, and I sent that to our wedding coordinator. I tend to be pretty indecisive though, so I had to figure out a way to try to eliminate myself second guessing all of my decisions...this seemed to help!