if you're only having 2 days, then you'll probably be best served taking the ferry from boston to p-town, especially if you want to relax - driving to mystic from boston is about 1:45-2:00 and driving from boston to the central cape (hyannis) is also 1:30-2:00, depending on traffic. driving to p-town will be the better part of 3 hours, again, depending on traffic.

p-town has great beaches and bike trails, so you can certainly relax. you can also stroll along the town/waterfront to sightsee. for lodging on p-town, take whatever is available as july is the high of the high season. use expedia or travelocity to get an idea on openings, then call the hotel directly.

enjoy!