Cayo Largo is a small island, not sure how much you can see. Some resorts are too small to have scooters, may not even have bikes.
Tip in CUC for sure, leave your maid 1 CUC on the pillow.
Might check the resort for hurricane damage before committing. A small island is not the best place to be during hurricane season. Can read reviews at
http://www.tripadvisor.com/
Does $400 for 4 nights include tax? Check your resort cost and others at
http://www.redtag.ca/. Seven nights are about the same cost.
Think it is worth it to pay a bit more to be at a better resort, better food, room, free water, etc. A 4 star resort might be another $100.