Opportunities for outdoor activity are pretty easy to come by in Oregon. In fact you kind of have to go out of your way to avoid them. But if you are looking for some summer fun then Anthony Lakes should be on your list.
The trip can be a bit on the long side, so plan to stay for a few days. Take I-84 to North Powder and exit. Then just follow the signs. If you have no idea where North Powder is, it is more or less near La Grande.
There are a couple of lakes up there and plenty of camping. There is a small charge to camp, but nothing a poor man can't afford. In addition if you really hate roughing it there are cabins available as well. There are several trails for hiking, all leading to some really fun views. The fishing is generally good though you should double check the stocking schedules just in case. Canoes, kayaks and swimming are all a ton of fun but motorized boats are not permitted.
It is pretty high up, so don't plan on going anytime other than summer and even then you should plan on cold nights and cool mornings with plenty of sun in the afternoons. Bring your sunscreen and your winter coat.