Try using Google Flights (open the Google browser, type "flights" in the search box).
Try typing in destinations you think would be fun, as well as give you enough mileage. Just make sure that you're flying on United aircraft. If you're flying with a Star Alliance carrier instead, check United's website to know exactly how many miles you're actually earning.
United has a calculator on their website that shows how many miles you'll earn based on flare class. Happy trails!