Breweries, wineries, or larger restaurants often have a party room you can rent. Is there a favorite restaurant that you have near the hotel? Check with them first! Otherwise, could you rent a conference room at the hotel to hold the party in, and have a nearby restaurant cater?
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That is right on the "new" waterfront in Boston, an extremely expensive area. Are you willing to pay a huge amount for an engagement party? The parking will cost as much as a meal, near there. Why does it need to be near where the wedding is? Where will the people attending the party be coming from?