The thing that senior dogs and puppies have in common is their appreciation for sleep. As a puppy, they've spent their day running around and just in general being lovable. Older dogs have been there and done all of that. They're still active, but years of activity have brought them to a place where they only need a few short walks to get their daily exercise. Their joints are a bit more worn down and jumping around is not as enticing as it once was.
The best investments that I've made is to provide a comfortable place for my senior pal to lay down in the rooms where I'm most likely to be. He'll follow me from room to room, so I want to make sure that he's up off of the cold floor, and his old bones have enough support to keep him comfortable.
The most inexpensive option is a heavy quilted dog blanket. I picked up one of these from Costco as an alternative for travel, as well as a "third" bed. It's not perfect in that I need to assist fluffing it up frequently.
The first two that are in my living room and bedroom are full on dog beds. For larger dogs these can get expensive if you go to a specialty pet shop (reaching over $110), but there are plenty of great alternatives that you can pick up on the cheap if you shop around on the web. I find that my dog likes his bed firm, so look to see if the bed has orthopedic support to cradle their bodies and keep them from laying flush on the floor.