Are you an only child? If not, then enlist your siblings. Even if they live far away, see if you can get them to come over even once per quarter (every three months) or so. Even a short visit is something to look forward to. If you are an only child, then work on your cousins within your age group, or even your parents' own siblings if they are well enough to travel.
All of that having been said, one thing you can do is to invite them over on occasion. Maybe even do that as a corollary to telling them you're going to do such-and-such. For example, "Mom, my family and are going to the zoo this weekend. It's going to be a wonderful time and we're sorry that you and Dad wouldn't be able to come over on such short notice. Can you maybe come over for a few days next month and we'll go to the art museum?"
You don't have to invite them every time, or to everything, but it would go towards making them feel less isolated. And, the more everything is planned, the less time for uncomfortable times with them.
As for their drinking, of course that is troublesome. Do you have their doctors' names and phone numbers? If not, ask your folks for them. It's not for anything sinister, you just care about your parents and want to be sure that you have all updated information, e. g. in case they come over and someone trips and falls on the front steps.
Then, contact their primary care physicians and express your concerns. The doctor(s) may be able to talk to your folks in a way that you can't, for example to explain to them that they aren't at death's door so they should still do their best to care for their bodies and not let everything go to pot, and also explain how dangerous drunk driving is (I assume, of course, that your parents are well aware of that, but sometimes it's good to have such things reiterated by a caring, somewhat impartial professional).
Are your kids (I am assuming you have at least one) old enough to write to or call Grandma and Grandpa? My nephew (I don't have kids) is in a lively correspondence with both of my parents. They all treasure one anothers' letters.
Another thing you can do -- if it's possible -- is create a plan and stick with it, a schedule, if you will, such as "Dad, I'm going to call every week on Wednesday from work. I can't talk too long as I'm at work but I care and want to check in and be sure you and Mom are okay -- and I want to let you in on the latest news from junior. He'll/she'll fill you in on the details in our weekend call (or in junior's next letter), but we're very proud/happy/excited that junior has made the soccer team/gotten an A in her Spanish class/volunteered at the library/done such a good job caring for her new hamster/helped out at the community center/cleared a patch of land for his very own garden next year/etc. etc. etc." You'll think of something, I'm sure. And even if you don't have kids, there's got to be something to talk about, even the weather.
Keep it bright, nonthreatening, noncontroversial and upbeat.
"Dad, did you see how the local team is doing?"
"Mom, I was thinking of making your lasagna this weekend. Do you know the recipe offhand?"
"Dad, the disposal keeps getting clogged. Got any ideas of how to keep that from happening?"
"Mom, we're thinking of buying new curtains for the dining room. You know the colors in that room. What do you think would work best?"
Or something like that. These are your parents, you know them. And they want to feel needed and loved and useful.
You don't have to do this alone, but they are somewhat dependent upon you, so you may end up having to be the one to make the first move in this area. Yes, it's time out of your life and no, it's not simple, but this is the time to be in touch with them, as there will come a day when it's only going to be memories.