I have the same resolution as you, and here are a few things about which I have thought so far :
First of all, I will try to avoid going by plane on vacations. If I do have to use a plane, I will try to go with an eco-friendly airline that uses biofuel (like United) and try to use a direct flight. The same way, when I have to go in train, I will get some informations about the trains, to see if I can choose an eco-friendly train.
When you go to a hotel, you can also find informations about the place. More and more hotels have eco-friendly policy, and I prefer to spend my money in these hotels than waste it in a huge touristic and polluting hotel. These eco-friendly hotels usually have certifications, so don't hesitate to check these certifications. Also, it is best to choose a locally owned hotel. Another trick to know is that in general, the all-inclusive hotels aren't sustainable. You can also go camping if you can, this is the best for the planet, and it is an amazing experience !
When you book tours or participate in activities, once more, check out the policy of the place : try to support local economy. Try the local food of family restaurants and never go to McDonald or that kind of multinationals. Don't be (too) afraid to eat in the street. Also avoid the activities that might abuse animals or destroy the environment. For example, all the places like Marineland are a no-go for me. I also avoid riding on holidays, because you can never be sure of the life-conditions of the horses. In general anyways, the activities that involve "having fun" with animals aren't sustainable : it is best to avoid walks on elephants' back, all the zoo, or the activities with monkeys. In general, I always strive not to disturb the wildlife (on the land but when I am diving as well), even if it is often tempting to interact with exotic and cute animals. Also for what is about supporting the local economy, enjoy your trip to buy gifts made by local artisans. You will then be able to give these original gifts to your family and friends for christmas or their birthdays.
If you have a zero-plastic and zero-waste way of life, keep up the good work even on vacations. Do not give up on your life-principles just because you are not at home. It may seem like an obvious resolution, but I myself used to use way more plastic when I was on holidays than in my daily-life. Also, you should continue to limit waste of energy and of water, even on holidays. For instance, I only buy bottles of water if I absolutely have to (when I am in a country in which I can't drink from the tap). Also, try to pack lighter now, and to go abroad with eco-friendly products.
To finish with, something that I always care about is to avoid green-washing : a lot of company now understand that consumers are interested by the sustainable development and lifestyle, so they promote as sustainable things that aren't. To avoid this, I always double-check what looks "too good to be true", and I try to get opinions from other customers. And I also give my opinion and promote the places, hotels and restaurants that I liked to help other consumers ! :-)
I hope this was helpful ! If you have other tricks to travel more sustainable, please share them, it would be really helpful !!