Two Tickets To Adventure by Pravit Agrawal

When Jon and Rafe go on a holiday to meet their grandad, they expect to kick back and relax but their mother decides that they must be actively moving and exercising. Jon and Rafe are forced to go on a beach walk, when they stumble upon a portal which only they can see.
Upon entering they are faced with many challenges and obstacles to overcome in this alternate world. Will the brothers get back safely? Who knows?