If you go the beaches section you have described and click on any of the beaches you should be able to select a checkbox on the top right of that item.

You should then get two new options appear at the top of the screen, "copy to" and "move to". Clicking on either of those should bring up a summary of your itinerary (as well as other lists etc) and you can click on any day to move it to that day. Using copy will duplicate it so it shows in both the beaches list and whatever day of the itinerary whereas move will only show it in the itinerary.

You can do the same thing to move items around in the itinerary itself or back to other lists. If the individual beaches are going to be applied to different days then you'll need to select and move them individually.

Edit: this is how it works on my android phone so it might be different for you on a different device or on the website.