I personally have let them run around the room for as long as they like(but I also work from home so it's easier for me to have them out 12 hours a day). I also give several types of fruits, veggies, and any sort of treats throughout the day(in moderation of course). Personally I've found when they're about to pass they just like to spend time with me and their other cage mates as much as possible. If the cage mates don't want to cuddle and instead play I'll hold the rat and comfort them. I had one of my rats refuse to let me stop petting them and they passed away overnight. They want to be with someone they love during these times.
And I personally let them indulge in foods I wouldn't normally give them, like crackers and sugar water. But some people might consider that bad 😅
Give them toys that give them fun, like a dig box if you don't already have one. Puzzle toys, Pinatas, tubes And tinnels. Rats also like mint plants. Maybe buy a mint plant(they will destroy it).
Keep them warm, their metabolism isn't what it used to be.
Take them outside, build a rat proof pen in the grass and sit with them.
Have bells in the cage(my rats LOVE bells).
Some of these experiences you can give to your young rats too. They'll appreciate it.