If you like shopping, take time out to go to Agritown when in Obu. It's also full of great shops and restaurants, so you can spend the afternoon having a wander.
Sightseeing doesn't have to be expensive. You if want to get outdoors, try a visit to Aichi Fitness Forest Park or make a stop at attractions like Okura Park. Don't forget to include a visit to Agritown.