The phrase "up tree" can be replaced by a number of synonyms, depending on the context in which it is used. In a literal sense, one might say "climbing a tree" or "scaling a tree" as an alternative. For more playful language, the phrase "up a tree" can be replaced with "treed" or "perched". If the tree in question is a fruit tree, phrases such as "picking fruit" or "harvesting" could be used. For a more elevated tone, one could say "among the branches" or "in the canopy". Whatever the situation, language is a versatile tool with which to express oneself in a myriad of ways!