If you manage to get yourself some fruit from another town (i.e. if your town grows cherries and somebody gives you an orange or pear) then you should plant it right away. This is a great way to make cash.
To ensure that your fruit will grow you should first chop down a native fruit tree and then plant the foreign fruit into the hole left by the old tree. Each new fruit that the tree produces will be worth 500 bells which is a heck of a lot more than the 100 bells for native fruit.
Over time this can build an orchard that is producing tens of thousands of bells each week.