My almost three year old still naps (in his crib). I think kids generally stop napping when they go to a bed cause there is nothing keeping them there. So the first issue is .... keep him in a crib as long as you can! Even if baby number two comes along!
One thing we do during the day is have "blanket time." My son loves it! I lay out a blanket on the floor and we do something together on it like puzzles, read books, toss card (from a deck of cards) into a laundry basket, pour large beans into cups, play quietly with any toy......
The rule is "on the blanket is YES and off the blanket is NO." So eventually and even sometimes now I can leave him to play on the blanket alone while I do something else in the same room. I remind him to stay on the blanket if he gets off.
The whole point is to get him to learn the self control and to obey my instructions. Then when it comes time to switch to a bed - the same principles apply. "Stay on your bed - if you need mamma you call for me but do not get off your bed."
You get the idea.
I think you are wise for thinking ahead. The best parents think ahead and have a plan and begin training for the next step. These are valuable days that you can be training him and setting him up for successful transitions.
Off topic but same idea: Potty training. Start now - not sitting on the toilet but start having him learn to pull his pants down. When you need to change his diaper have him pull his pants down, when it is time for a bath let him pull his pants down. He will have mastered it by the time he NEEDS to do it quickly.
THAT IS PLANNING AHEAD! And will make the whole transition much smoother.