We spent five nights/six days on the lower Sunshine Coast in the spring a couple of years ago and are contemplating doing the same again this spring. With 8 nights, maybe you might want to visit both the lower and upper SC. We found plenty in the way of hiking and biking to fill our time, and wanted to go on a boat tour one day, but the timing didn't work for us. We also didn't do any kayaking; it was too early in the season for that. I guess what I am trying to say is that even on "just" the lower SC, if you are interested in outdoor activities, there is plenty to choose from. But on the other hand ;-) , eight days is also enough if you wanted more of a "road trip" style of holiday (and the ferry fares are within your budget), you could also do a route up the SC and over to Vancouver Island, then back to Vancouver.
I can't offer any advice about how busy it is in July and whether booking ahead is essential (I think it would be wise, unless you have a budget that can accommodate "surprises" if the only lodging left is pricy). Partly it would depend on the type of accommodation you want to stay in - if you want to stay in B&Bs or vacation rentals, then you'll need to book ahead. We stayed in a B&B suite in Halfmoon Bay - we wanted to self-cater and chose this location for nearby hiking, and because it's only about 15 minutes to Sechelt (for groceries and for the pool, DH was training for a triathlon later that summer and needed to maintain his workout schedule even while we were on vacation). We also enjoyed the Saturday Farmers' Market in Sechelt. If we go back again this spring, another essential stop for us is the Sunshine Coast Olive Oil Co. in Gibsons.
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You definitely want to pre book your accommodations. July is peak season. The entire lower coast (Gibsons/Sechelt areas) only has a few motels, it is more cottage rentals. Upper coast (Powell River and areas) is about the same actually. Winging it is not recommended.
8 days is plenty of time - what are your interests? What are the ages of everyone? Have you been to Vancouver Island before? As krp mentions above you might consider the loop but you would need to plan the ferries and accommodations and stick to the schedule to make the most of your time.
There are plenty of accommodations websites for the Sunshine Coast: Just google and start your search.