This happened to my son, who was 15 when diagnosed, at times. It seemed that once he was over 250 or so, it took a lot of insulin to bring him down. He became very insulin resistant.
We found the best thing was to take him to a big box store, buy him a bottle of water and stroll around the store looking at the electronic gadgets, games and car parts. He enjoyed this more than walking the neighborhood with his Dad or I which he said was way too embarrassing, especially with us carrying a bag with his tester, glucagon pen, cookies, etc just in case the insulin suddenly kicked in!
The easy walking always brought him down whereas street sports with friends seemed to make him higher due to the neverending competition at that age.