I'll be staying a week in Cape Cod next summer, and want to start planning my trip early. At the time I go, my kids will be 3 and 4. We are planning on renting a house/condo/cottage for the week. We prefer a vacation that's 50% relaxing (beach/pool), and 50% activity (sightseeing/biking/walking). What town is a good place to stay where we won't have to constantly be in the car to get to things that we want to do or see?

Wellfleet is where we vcationed every year when my son was growing up. The bay beaches are great for kids as well as the ponds, it is very bikeable, near to Provincetown and has wonderful restaurants and galleries.

The Commodore in West Harwich has a pool and is seconds from the beach. Many nearby kids activities.

With kids that age, you'll want access to the bay, the slund andonds for warmer, and calmer water.
Skaet Beaxh in Orleans is great for little kids , Pligrim Lake.
Any beaxhes on the Sound will be good also.

It is great to find a pond for little kids. they might like to go to Oyster Pond, a saltwater pond, near downtown chatham. They have new bathrooms there and the parking is free. It is known as the childrens beach in Chatham.

Thanks! All great suggestions.

It is hard to "pick" a town on the Cape regardless of kids or not. I would do some research of your own to get a feel for what each town has to offer. Also think about where on the cape you would like to base yourself. It takes one hour with zero traffic to get from Hyannis to Provincetown, in the summer you can bet on traffic.
Do you plan on exploring the cape or will it be a beach,pool vacation? Since you have alot of time you should be able to find a nice place to stay.
My favorite beach for kids is Skaket beach without a doubt.This beach is in the town of Orleans and you need to get a parking sticker from the town. The towns next to Orleans would be Eastham and Brewster, this is a fairly central location depending on what you would like to do.
You really can not avoid being in the car on the cape, it is kind of spread out with many beautiful sights to see. The outer cape is beautiful, Wellfleet, Truro, Provincetown all worth a visit.

If you want to be in a less touristy area I recommend renting in Hyannisport or Centerville. Falmouth and Dennisport are good for biking and walking because they have designated paved trails just for that. Are you flying into Boston, if you are then Falmouth is closer. If you fly into Hyannis, then Centerville and Hyannisport are literally down the road a few miles. Dennisport is very touristy and is a nightmare to drive to from Boston in the summer. It would take up to an hour longer driving to Dennisport than it would to Falmouth and maybe 45 minutes longer than Centerville and Hyannisport. Any farther than Dennisport there is only Brewster which has Nickerson State Park which is very nice to go to for the day but quite a drive. The South facing side has much warmer waters and rentals will be much cheaper on the North side but there is very little to do there other than Sandy Neck Beach. Rent early because the good places will be gone soon. A visit to Provincetown and/or Chatham is definitely recommended for the day or evening and are all great places to eat and buy souvenirs/ walk around an enjoy the ocean air. Good Luck