No objective "fun" metric exists, but traveler surveys, repeat visitation, and cultural vibrancy consistently highlight these five:
Carnaval, samba, 7,500 km of beaches, Amazon, Pantanal, caipirinhas. Joy is a national sport.
Islands (Phuket, Koh Phi Phi), street food (pad thai, mango sticky rice), temples, full moon parties, $30/day backpacker luxury.
Maya ruins, colonial cities (Oaxaca, San Miguel), Baja whales, Pacific surf, Day of the Dead, world-class tacos.
La Tomatina, Running of Bulls, Feria de Abril, tapas crawls, Gaudí, Alhambra, 300+ beach days/year.
Rome, Florence, Venice, Amalfi, Sicily. Art, wine, pasta, gelato, passeggiata. Life as aesthetic practice.
Best approach: Define your fun, then match.