Newbies, Cape Cod w/ small Dog, mid-May

We've stayed at Sweetwater Forest a few times and are going back this year, but it's not near the water.
Yes, bring your bikes. +1 for the Cape Cod Rail Trail, a paved multi-use trail that mostly follows an old rail bed (no hills.) If you're on the trail, stop at Arnolds in Eastham for lunch.
I believe there are a few places you can rent kayaks from but you're on your own to find them.
The Salt Marsh Visitor Center (National Seashore) offers guided canoe trips into the salt marshes. You can also park at the Salt Marsh VC and bicycle to one of the beaches.
The Cape Cod Museum of Natural History is a nice place to visit, you can walk the trails to the ocean. In Brewster check out the general store and have some ice cream at the shop next door.
Provincetown is an interesting visit for shopping, dining, and experiencing the alternative lifestyle. Parking can be a PIA though.
If time and budget allows, take a ferry to either Martha's Vineyard or Nantucket. There's a privately-run whaling museum in Nantucket that has an exquisite collection of scrimshaw, and IMHO that museum is better than the National Whaling Museum in New Bedford.

Atlantic Oaks is nice, a bit up in price. Also look at Sweetwater Forest in Brewster, very central to the cape highlights, close to beaches.

Nickerson State Park is truly awesome, but no hook-up, central water and showers. Excellent hiking and biking, fishing -- and if you bike or hike the Cape Cod Rail Trail is great, flat, paved trail runs for miles and miles thru cape towns and forests.

We lived on Cape Cod for about twenty years - so if you have specific Q?'s, we welcome a PM ... you'll love the Cape in May - no crowds !!!

Make sure you visit the Heritage Plantation in Sandwich ... and tour the MassWildlife fish hatchery in Sandwich as well.

IF you like Jazz - find Bruce Abbott, an incredible Sax player ...

ON EDIT: Dogs OK on beaches until mid-June, with normal clean-up rules.

We spent a couple weeks at Cape Cod Campresort & Cabins 2 years ago. It's at 176 Thomas B. Landers Road and has an East Falmouth address. Great place. Couldn't ask for more. Maybe 3-4 miles north of coast.
http://capecampresort.com/

Are you planning on renting a car while there, the Cape is pretty spreed out and if you're a newbie In don't think driving a 30' rig around daily is a plan for fun. The Cape is great we spent the summer in 2014.

Nickerson Stste Park is really nice. Has a couple good lakes but also a two minute drive to the ocean. It's located mid- Cape so it's fairly central to everything. If I may make a suggestion, hit up the Chatham Pier Fish Market for some of the best seafood you have ever had in your life. Also watch the sea lions beg for a treat as the boats come in to unload the days catch.

Try Peters Pond Park in Sandwich. They've been under new ownership for a few years and reviews are much better. Not far from the canal and a number of beaches. As far has finding a beach for the dog, not likely. At that point in time most of the local beaches require permits and the national seashore beaches won't allow pets. PPP has a pond on the property but I don't know if they have a dog beach.

On cape cod, you're never far from a beach! We stayed at Atlantic Oaks a few years ago and really liked it. It was extremely clean and well kept. It backs to bike "rail trail" which, by definition has no steep hills.

We stayed after Labor Day and the cape was nearly deserted. I would assume mid May would be similar.

Have fun!