Sometimes you'll get a request not for a specific item, but for any fruit, bug, shell, fish or flower of your choice. Make sure you give out the items that are worth 10 bells only! You'll still receive a gift and friendship points for doing so. Giving out higher items does net you a higher reward, but never give away anything worth more than 1500 bells, as the animals won't ever pay you higher you than that. Giving away special fruit (worth 600 bells) can end in a reward of 1500 bells, and some rare fish and bugs can also end in higher rewards. But if you nab a fish or bug worth 2500 or more, just sell it directly.