It is not easy to approach this subject of giving money to your parents because you have it - It could be interpreted as showing off, rubbing it in their faces. Also, if your parents take pride in taking care of themselves, they could decline it.
I think that one of the best ways to give money is to give it as a gift for a set purpose.
For example, give them a gift of money for taking a holiday/vacation together- maybe with you, maybe without you? I noted that you said that they do not own a house or go on vacations. This would be a great opportunity for you to give a vacation to them.
Hello (mom) and (dad), I wanted to express to you how much I love you and how grateful I am of your support, and I wanted to give you a gift- please take this money so that you can go on that holiday/vacation together! It would be a wonderful gift to me if you go and enjoy yourself, and I do not mind at all.
Another case is that you want to support their living costs, along with what they have.
Hey (mom) and (dad), I wanted to know if I can help with paying X or Y bill? I know that you both are able to, and I do not want to take over any of your finances. The thing is, I am so grateful of your support when I was going through school, that I want to give back to you. May I help with this? I want us all to grow closer, and in this way, I feel I am closer to you because I love you very much.
Noted also is that they do not own a house. Do they want to? Maybe you can discuss with them about helping to put aside money for buying a house for living in and/or for vacation? This would show them you care very much and want to see them cared for well as they cared for you, because you are able to now.
If they decline these points, maybe treating them to a nice dinner out, or give them gift cards for special things they like- a spa day for your mother, a favorite sport game tickets for your father. Those are only general ideas to get you started. Maybe ask them what they would like for their birthday or the next gift-giving occasion?
Emphasis on being grateful and you loving them and wanting to give back is important here, so that they know that you are giving in kindness, and not in the intent of taking over anything. If they are not up for receiving money for the everyday bills, then the holiday/vacation option is a good way to give back.