This is going to go against the general advice offered up earlier in this thread.
Summer weeks at the non beach front Marriott HHI properties cost 1,400 Marriott DC points. That means you could rent DC points from a Marriott DC owner (having them make you the reservation) for about $750 to $850 for a week. That is less than the annual MFs for the same weeks.
The non beach Marriott properties in HHI are a much easier trade than those on the ocean. I have read instances of people trading their 1BR and studio weeks in to these properties, even in the summer. Chances are higher if you can travel the last few weeks of August or in June. I would think it would be fairly easy to trade a 2BR unit from any season in to the non beach HHI properties, even summer. You might be disappointed some years though. If you are flexible and can put in a trade request 12 months out for any summer week, you will get a trade. If you limit it to just a few weeks of the summer, you will be disappointed more often than not.
All said, I probably wouldn't buy a HHI property to accomplish this. If you are flexible, buying a lock off property elsewhere will give you more flexibility. If you can score a island HHI summer with just one half of the lock off, your price is effectively only about $700. If you want summer in HHI, just use the entire 2BR unit, but the effective cost is much higher.
Though if you are wanting to travel in summer to one of the non beach HHI properties pretty consistently. Just rent a week through someone's DC points account for less than any Marriott MF.