We are planning a trip this spring for 2 active couples in their 50's. We plan to fly into Phoenix and drive up to Flagstaff. When is a good time to visit?
That depends on your weather and activity preferences. Flagstaff can see snow into April (and May is not unheard of), although it is generally light and short-lived at that time. If you prefer cool temps and don't plan to do a lot of hiking (the ground can still be muddy), etc. then either month would be fine. Ideal Flagstaff (and Grand Canyon) weather, in my opinion, begins around Memorial Day.
Late September/early October is my favorite time of year, not only weather-wise, but crowds tend to thin out at that time of year as well. April can still find spring-break travelers about. As for the weather, temps are cooling, patterns tend to be fairly stable (aka mostly sunny) and it's pretty unusual to see winter weather make an appearance that early in the season. You also have beautiful fall foliage in the mountains around Flagstaff, Sedona etc. to compliment the views.
Priority #1, after deciding when to visit, is to get lodging booked. Staying as close to the park as possible is preferable to most, so those hotels are always first to fill. For Grand Canyon Park Village (in-park) visit www.grandcanyonlodges.com or call 888-297-2757. Tusayan, aka Grand Canyon Village South, just outside the park, can be booked here on TA or on Expedia, Travelocity, etc.
Using Flagstaff as a "base camp" for a few days is a perfectly viable option too. That opens up more possibilities for better hotels, but the disadvantage is that you face drive times of 90 minutes to 2.5 hours (one way) to get to the Grand Canyon, Lake Powell, Petrified Forest, etc. Here's a short list of some of the better Flagstaff hotels with promo codes ->: http://bit.ly/W3g0Ov
Flagstaff is a 90 minute drive from either park gate. If you choose t.o use it as a base for visiting GCNP, you'll pretty much miss sunset and sunrise viewing at the canyon.
Choose instead to stay inside the park, even if only for a single night.
Other destinations nearby from Flag would include Sedona (1hour) and Page 2(hours). Again, IMO, it makes much more sense to stay at these destinations rather than try to commute to and from them. Driving in the darkness in northern AZ is highly discouraged
I'd suggest you check out Sedona, AZ as it is on the way to GCNP. It will take a little longer driving time but worth it! Take I-17 to 179 (Village of Oak Creek exit) to 89A North and drive up Oak Creek Canyon to Flagstaff, then the GC.