You can give the guards Fresh Water, Soda Pop, or Lemonade to quench their thirst. You can get drinks on the top floor of Celadon's department store. That'll grant you access to Saffron City, where you have "two" gym challenges awaiting you, among a couple other goodies!
EDIT: Oh, and don't worry about that Power Plant quite yet. From what I remember, that's completely optional.