Josh,
Actually look into a decent quality bike from a reputable bike shop. As mentioned above, Wally World employees are dangerous folks when it comes to bikes and such.
A sandy area lends to the plus bike. 27.5 or 29 x 2.8 or 3.0 would make floating a breeze.
Now the recommend… I recommend test riding a number of bikes in both 27.5 and 29 to get an idea of what they ride and feel like. Also, if you find one you like, ask if they will rent for a few days or a week. Typically, a shop will do a rental and apply the rental fee to your purchase if you buy it.