I definitely think they're worth it. In my experience, open days show you other options that you may not have thought of or found out about otherwise. And since you've already booked your flight, you might as well make the best of it. Check up about everything from the different courses that cater to your abilities and interests, as well as scholarships, financial assistance (which is not exactly the same as a scholarship), accommodation, etc. It's also a good idea to do some preliminary research so that you can ask any questions you have to the experts who will be there (lecturers, student ambassadors, etc). But the most important thing is to keep an open eye of other opportunities or pathways to consider.