I assume you mean July rather than June?!
Falmouth is lovely, and well worth taking the time to explore up the river to Truro on a high tide. And a visit down into the Helford River, which is really nice and peaceful. I really love going into the little harbour underneath St Michaels Mount, although given that's National Trust I don't know if it will be open for visitors then or not in the current climate.
Assuming you have a petrol RIB, are you sure Newlyn Harbour has petrol? Last time I was there in the sailing yacht a couple of years ago, I thought it was only diesel from the harbour, and petrol you'd have to take cans up to the local garage.
Scilly is absolutely stunning if you've never visited, definitely helps to have good weather though! Moorings in Hugh Town are typically very bumpy, personally I avoid it in all but flat calm weather, very easy to get to town from there though. Porth Cressa is slightly better in some conditions, but one of the most protected spots available is between Bryher and Tresco in New Grimsby Sound. There are moorings there, or if you can dry out, Green Bay on Bryher is a nice flat sandy bay to anchor in - we've spent many nights on the sand there on our sailing yacht (which has a lifting keel!). All the islands offer something slightly different, we love St Martin and Agnes/Gugh, and the outer/Eastern islands around Great Ganilly and Menawethan are probably where you'd find the most wildlife - typically loads of seals around there. Do be slightly careful with navigation between the islands - loads of shallow areas and rocks, but as long as you read the charts and check the tides, you can safely visit everything.
Falmouth to Scilly is about 60NM from memory, so a reasonable passage in what can be fairly rough open water, if you want to go there and back in a day. Obviously dependant on what you/your passengers enjoy - they may need to be prepared for a bumpy ride!
I did a short video of one of our (sailing) trips here: