Looking to go for 7-10 days mid Sept. Enjoy beaches, wildlife, and just strolling. Where to look for good places to stay along the way. Flying into Miami and renting a convertible. Suggestions of where to stay, reasonable prices?

The beaches on the Keys are mostly not very good. There is a reef (lots of snorkeling/diving from boats) a few miles out which breaks up the wave action for beach creation. There are a few good ones--Bahia Honda state beach is a nice one about 2/3 of the way to Key West, no accommodations. The Moorings resort cottages in Islamorada (not inexpensive) is on extensive grounds and probably the best beach in the Keys.

Ditto the beaches in the keys. The keys are coral and all the beaches are man-made. If you're looking for a beach vacation, try the panhandle or either coast.

In addition, September is the height of hurricane season.

I would spend about 1/2 your time in Islamorada and the other 1/2 in Key West. In Islamorada, go snorkeling, kayaking, have some great meals. Then head to Key West. Stop at Bahia honda as described above. Then on to Key West.
In terms of accomodations, take a look at tripadvisor or other site and see what looks good to you. It's easier to respond to what you think is reasonable if you give some suggestions of what you are thinking of.
You may want to spend the last night in Miami.

If you want a beach vacation with a Caribbean feel like the Keys, check out Sanibel Island. It is on the SW Gulf Coast, off Ft. Myers. Beaches are very nice.

Ditto xkenx and SusanCS on the beaches in the Keys. The few beaches in the keys which are nice are located at Bahia Honda State Park & Long Key State Park. At Bahia Honda SP you can rent a kayak, swim on either side (but sadly no waves), snorkel from shore or take the snorkeling boat trip - its awesome. There's also a hiking trail near the lagoon.

If you are flying into Miami, Miami Beach & Ft. Lauderdale beaches are really really nice.

For wildlife, Everglades National Park.

A terrific day trip from Key West is the hydrofoil to the Dry Tortugas National Park. http://www.nps.gov/drto "Between the months of March and September, some 100,000 sooty terns will come to nest on the islands of the Dry Tortugas. They are joined by brown noddies, roseate terns, double-crested cormorants and brown pelicans."

Just keep in mind, mid-Sept is the height of the hurricane season, and there is only 1 main road through the keys.

"Enjoy beaches, wildlife, and just strolling" then maybe Key West isn't for you.

Sanibel Island on the gulf coast is quite nice too.

I agree with the Islamorada and Key West recommendation. The Keys offer every kind of accom from scary to unaffordable. The Hampton Inn in Islamorada is a good mid priced place. Nice pool, walking distance to some good restaurants, even a little beach with wading. More expensive is the Hilton in KW. Adjacent to Mallory, on the water. Beautiful cafe. As you are off season, you might check out Tranquility Bay condos in Marathon. A couple years old, on the water, nearby the old pedestrian bridge out to Pigeon Key. Have fun.

The Hilton in Key West sold to the Westin a year or so ago. Still a great location, though.

In Key West, the two most reasonably priced hotels are the Best Western Hibiscus and the Southernmost Hotel. Both get excellent reviews. There are all sorts of small inns and B&Bs that are less expensive, too. You might take a look at keywestinns.com