First of all, let me applaud your club coaches who said to wait until her sophomore year before reaching out to collegiate coaches; that is refreshing to hear!!!
There are many options available to improve a player's skill sets during the summer months and each one presents a different opportunity. Of the ones that you mentioned, I would suggest private lessons and sand volleyball.
Private lessons allow you to specifically target the skill sets she would need to be successful at the upper DI level; footwork, armswing, blocking mechanics, etc. Private lessons also allow for a best use of time, as they can accomplish quite a bit, in a short time segment and hopefully based upon your schedule/calendar needs.
Sand Volleyball is all the rage now with club and potentially college volleyball (sand), but many folks forget that it is also one of the best training and skill improving opportunities there is. Playing doubles in the sand forces players to develop their all around game (a big criticism of today's college player and one reason that international players are so desired by elite teams), teaches strategy and competitive focus, and is in a physically safe environment (the sand is wonderful on the body; makes it stronger without impactive stress injuries).
College camps tend to be very expensive and who knows who the coach will be instructing your daughter; could be good, could be sophomore, could be a buddy of the college coach.
Club camps are also a bit unpredictable; who is doing the instructions? Is it a guest coach or is it the same coaches she has already had? Is she going to improve in the same environment or will having new stimuli be better (yes, it will).
As this is your PSA's first full year in club, and she is tall, she will have the potential to improve to that elite level, but it may take her a bit longer than a freshman who is 6'3" and has been playing club for a number of years.
What you want to be careful about is pushing too hard too quick; yes she is probably rapidly improving at this time segment but this rapid improvement can be physically and mentally overwhelming. Please make sure to build in plenty of down time so she can just be a kid!