May - I have spent many days on the beach in the hot sun, the only negative is that the weather's unpredictable and the sea temps are only around 18-19c, brrr, too cold for this big softie!
Anyway, this is what I'd do,
Land at the airport and have a snack in the bar. Then a taxi (if the queue's not too long, could be quicker on the bus!!) to Ses Salines/Es Cavalet on the south of the island, choosing which beach based on wind direction.
Sunbathe (hopefully!) for a few hours and then have early dinner at either of the two restaurants by the bus terminal, and then get the last bus back into Ibiza town for a few beers before heading back tot he airport.
If it's poor weather, then I'd simply head to Ibiza town, catch the ferry to Talamanca and back and try to keep busy!