All in one beach chair. And now you can get comfortable pads for even the cheapest lounges. This approach has a side benefit. Lounges with adjustable backs are okay for napping but not for sleeping. You will have to consider some sort of bedding.
Futons are great.
The good ones are both too heavy and too expensive. You only need it for three months. Or if you must have a bed, go to one of those mattress and furniture discounters near the shore. Tell them you are looking to furnish a guest room. You'll be shocked at what your hosts have been doing to you for years.