Established in 1903, Clark State Forest is the oldest state forest in Indiana and today covers more than 24,000 acres. Oak Lake is one of seven fishing lakes in the park (five of which - including Oak Lake - are also open to boaters) and there are picnic tables and grills both here and throughout the rest of the forest. Other activities include hunting, mountain biking, hiking and horseback riding. Primitive camping is available.
If you go:
A tornado in March 2012 caused serious damage, cutting a large swath through the forest. Most of the trails closed as a result of the damage have now reopened but the effects of the tornado will be visible for many years to come.
Links:

Clark State Forest
2 Service Drive
Henryville, Indiana 47126 USA
ClarkSF@dnr.IN.gov
Mailing address:
Clark State Forest
P.O. Box 119
Henryville, IN 47126 USA
Phone Numbers:
+1 (812) 294-4306