The Mario Park is a good day out, it is not as modern as some other but we enjoyed it, the water is salt water loads of slides wave pool, I would not book with the rep, as it's cheeper to get there and just buy a ticket at the gate
Take a picnic (although there are cafes etc on site) there are seperate picnic areas (you can not eat on the sunbeds) watch how the spanish do a picnic! it's a work of art.
Sunbeds are charged seperate (about 3 euros for two and a sunshade if I remember)
There are not massive ques so you don't have to wait ages for a go on the slides
Top tip, if you get a taxi try to book one from your hotel as at peak times (10 -11 am you can wait ages for one at the rank.
A nice touch is that the security gaurd will ring a Taxi for you to get home