Go for the Gyms

When you see a gym, you may be tempted to move on and focus your time on pokestops.
However, even if you can't defeat all the pokemon, battling and training at gyms whenever possible is a good way to earn experience, burn off extra items, and either raise or lower the prestige of the gym (depending if it's controlled by members of your team or not.)

For example, even if a gym is defended by multiple pokemon and you're only able to defeat just one, you'll still earn experience (and possibly medals) for yourself, burn off extra potions and revives when you heal your pokemon after the battle, and raise or lower the prestige of the gym.

Besides, what's a pokemon game without battling and training??