I think the first important thing is to let go of this idea that you are going to 'make' your parents do anything. This is not even what happens when you are a parent. You will of course give your child every opportunity you think will benefit them, but they are the ones who must decide that something sparks their passion and do the work required to achieve in that area.
When people get to the age your parents are at, having raised a family and passed many of the usual life milestones, and are not doing the things they may have wanted or needed to do to feel full life satisfaction, there are either deep-set reasons or ingrained patterns. It is hard to change a lot of things about who you are and how you go through life at twenty-five or thirty. It's even more so at fifty. If you go into this with the expectation that you will enact particular change, you may wind up frustrated and burned out and damaging the relationship you have with your parents.
I have been where you are, deeply grateful for the life my parents set up for me and wanting to help them achieve their dreams. My case was even clear-cut as I had a parent constantly talking about the things she had always longed to do and what she needed to do them. I was easily able to remove the barriers in front of her with financial and technical support... but a number of years have passed and she is not in a notably different place. She has found new intangible barriers to replace the practical ones. I think I have helped, but she needs to work through a lot of stuff in her own head before she can achieve her dreams, and that is something I can't help her with beyond showing a willingness to listen.
Offering to open up your parents' world through for example English classes is a great idea. They need opportunities to find new interests on their own and take ownership of any process of adding new things into their lives. By the same token it is only likely to be effective if they are motivated and not just being nagged by you into doing things. If spending time with their children motivates them you could find a class or other activity you or your sister could also attend. Or if you can work out what some of their unfulfilled goals might be, find or organise an activity that will lead them quickly to some small part of that goal in a way they can see, so that they may get excited enough to start pushing the process themselves.
But do remember that they are their own people, and you can't judge how they are using their lives based on your values and experiences. A mistake you can make when stepping into the 'parenting' role is trying to give someone what you think you would have wanted, which may be useless to them. Try looking at how your parents spend their leisure time without judgement, it may actually hold the key to how you can help them. Their TV-watching might indicate they would enjoy the opportunity to see more movies, or the shows they watch might tell you they are interested in food or sports. Even if they just watch terrible soaps that might indicate they would enjoy certain so-called 'trashy' novels! If they like to go on about politics, they could get involved in a party. That might be a good way to make like-minded friends, which could be all they need to fill their lives up. Socialising is a hobby for a lot of people
Certainly never let go of the idea of bringing positive things into your parents' lives, but realise that just because they seem to exhibit dissatisfaction does not mean they are looking to do things differently or that they will do so when you think they should. I have yet to go through this myself but I have been told that the point at which all the children are grown and independent can be a difficult time. They may simply be mourning the end of that stage of parenting and working through the realisation that they are a lot older now. They've clearly done some things right if they have a child who is motivated to give back to them. Let them know you are eager to help them with anything they need and, however hard it may be at times, respect that they have the right to use their lives as they choose.
