Is it safe to do slow exercise if my glucose is 250 mg/dl? i'm obese and i'm not on insulin?

Although a glucose level of "250 mg/dl" is not unusual, depending on the type of food intake, and time elapsed when the test was performed, see your primary care physician who may order a glucose tolerance test. Exercise is generally not a negative factor in any case.

Yes, modest exercise is safe. In addition, one should avoid High Glycemic index foods (as found in fast processed sugar foods and Fast Foods) - and One should, at least take "Oral Diabetic Medication" as prescribed by the PCP or Endocrinologist. A Dietician's help can also be sort.