I'm Miranda, and I'll be happy to help you with your question.
Booking travel information can be quite frustrating, and you will probably drive yourself crazy if you are constantly looking at every site to get a better deal. Most travel sites' prices don't change that dramatically from day to day, so while you might save a few dollars by spending an exorbitant amount of time every day searching the sites for a tiny savings, you might find it less frustrating to stick with whatever you originally chose for your travel arrangements.
There are so many travel sites out there (Orbitz, Expedia, Priceline, Kayak, etc.), and each typically has some sort of rewards system. If you have worked with them in the past, and they have an excellent rewards system for their customers, you may want to continue using them, even if another site might offer you a few dollars in savings. Those rewards add up, and the rewards you can earn may very well be worth more in the long run than the few dollars you may save by using a different site every time you make travel arrangements.
For example, I'm partial to using Hotels.com for booking hotels. Could I save more by using a different site? Possibly. However, Hotels.com has a reward system where for every 10 hotel night stays you book with them, you get a free room. Now, perhaps I could have saved $5.00 by using a different site every time I booked a hotel. But in the long run, 10 hotel stays using different sites would only save me a total of $50.00, where I could have earned a free $80.00+ hotel room by using Hotels.com to book those stays.
As for Groupon, the site does have some good, legitimate offers as you have seen. However, there are definitely some fake scam offers as well. You just have to be careful with what you buy. And sometimes the communication between Groupon and the businesses can get a little skewed. For example, I used to work for an outdoor activity center (ziplines, etc.) that used Groupon to offer packages with discounts for the activities. However, what Groupon stated was in the offer didn't match what the activity center had promised to provide. In other words, Groupon promised more than what the activity center agreed to. The activity center did end up honoring what the people had purchased, even though it was vastly different than what they agreed to, but they stopped using Groupon to offer discount offers after that. Now, that's just one example obviously, but it does show that Groupon has its faults. Again, there are some great, legitimate offers on there, but you just have to be careful.
As for their online courses, it really just depends. Some of the courses could be wonderful, while others may be completely useless. I would suggest looking up reviews for whatever business/university is offering the classes and seeing what people who have taken them have to say. Also, it depends on what the classes are teaching that can help determine their worth. For example, there may be a 12-week online video class for $150 on cake decorating offered on Groupon. That may be very useful if you want to learn how to decorate a cake, but you may find free videos on YouTube that can teach you the same thing. Or there might be a business in your city that may be offering an in-person, hands-on class to teach you how to do the exact same thing for less money (I know my local Hobby Lobby does this.).
Another thing to evaluate when you are considering taking an online class is how you learn. Do you learn better when you are taught by someone else, or do you learn better when you teach yourself? If you learn better on your own, I wouldn't suggest paying for a Groupon class on Greek mythology, when you could learn the same thing for free using your library and online research.
I hope that answered your question. If you have further questions, please ask. It's been my pleasure helping you. Please remember to rate my answer positively, as rating me is the only way I receive payment/credit for my work. If you would like to request my help in the future, please post your question with "For Miranda Only" at the beginning, and I will respond.