Some of the cutscenes before each level are a lot of fun, if a bit brief. Perhaps best are Congar's fight with the military right at the start, and Agamo and Magmo's battle towards the end.
Causing disasters during fights. Perhaps best is Baytown, which is left devastated by an earthquake, and Tsunopolis, where a tsunami flings any monster caught high into the air.
For whatever monster you completed adventure mode with. After taking on (presumably) every other monster in the world, you fight the alien leader, and beat the daylights outta him too!
Cities are almost entirely destructible. Have fun.
A good fight can be almost as fun to watch as it is to play, with buildings crumbling, monsters being flung across the cityscape and the devastation causing everything from earthquakes to volcanic eruptions, paired with all the cheesy, bombastic, awesome music.
Your monster of choice apparently thinks nothing of surviving a close-by nuclear explosion and then jumping into the smouldering crater to kill whatever it may find inside during the campaign.