We just got back last night. We had a great trip and Iceland is amazing. If you have specific questions, please let me know and I will do my best.
To start, let me address two questions I have seen in the thread:
Hiking shoes - Definitely take a good pair of shoes. We (my family) all had waterproof hiking shoes and wore them almost every day. We did not have shoes that covered our ankles and
ABD rented boots for us for the glacier walk. (Rented from the glacier walk vendor so super easy.) I don't want to assume this will be done for every group, but at the same time my guess is that ABD will make sure everyone is taken care of and has the appropriate footwear.
Blue Lagoon - Flip flops were provided. I will add that flip flops were not provided at the second lagoon. We took flip flops with us and I know that my boys used them but I probably wouldn't worry about them if I were to do the trip again. I forgot to take mine to the second lagoon after carrying them all the way to Iceland. Ugh!