for the gardens keep it simple and go for raised beds we have pic's etc.,. on our site on how we do that.

for the lawn well that is another issue, 2 ways to look at it though might be more sustianable to do some research on what grows best in sandy conditions it may not be like a regular lawn but it will be easily maintained.

other than that you would need to indorporate a good dose of organic matter into the soil first then top off with a reasonable layer of top soil, and keep adding finely chopped mulch each growing season.

or simply have more gardens.

len

Seaweed works very well to improve the structure and water holding capacity of the soil, due to the alginates etc.
Just get a few trailer loads that have washed up on the beach - you don't need to rinse it or anything. I suspect fleshy brown and red algae will give the best benefit. Just keep adding it as mulch year after year, and the results are as good as you could hope for.

Cheers,
Mike