My husband has been a SAHF for our 4 year old son since he was 4 months old. It has been the best arrangement for us. Sure, he and I do things differently. I would be a multitasker, arranging playdates, and some of the things stated by TJ in her response. However, he is so great at having unstructured play. They do have routines for lunch, nap, and other activities, but in between, they just play trains, ride bikes, play legos, etc. My husband gets tired of playing these same old games, but he is incredibly patient.
I would offer two pieces of advice: 1) It can be hard to be a SAHF. People expect men to have more of a contribution than raising kids. When he or I go out, people ask "What do you do?" and he responds he is a stay-at-home dad. Then, they say, "well what else do you do?" We reply it is a full-time job. People don't expect women to be a SAHM and have a job, but they do expect it of men. Luckily my husband does not get his sense of self-worth through an income or a title, so it does not bother him. However, this could bother some men.
2) Both of you should be very clear about your expectations. For example, my husband keeps the house clean (nap time is his time to clean up), but I do laundry and cooking. We talk often about how things are working and how things can be better. If I need him to do something, like start dinner, then I need to tell him and not expect it. If I prefer something to be cleaned differently, then I need to ask him about it or be willing to do it myself. If you think the kids need more playdates, then talk about it together. Appreciate what he does and that he will do things differently than you would. That may not be a bad thing...just different. If you expect something else, then talk about it.
Having my husband be a SAHF has been so great for our son. It is not always easy (neither is being a SAHM), but it works for us. Part of our success is that my husband chose to do this because he knew it would be best for our son.