Depends when you plan to do it, if this year then be quick!
Day 1: Start from Eastbourne or Newhaven (both have nice Marina and slipway), stop for Lunch in Brighton Marina, then head to Chichester/Portsmouth.
Day 2: Portsmouth to Ryde (IoW) for Lunch and then go straight to Poole (again nice marinas, hotels, B&B etc.)
Day 3: Poole to Weymouth, you will then pass the Lulworth Cove. Stop for lunch in Weymouth and then to Exmouth.
Depending on your rib, how much fuel you consume, and weather permitting you may combine that trip in just 2 days or even one and half.
If you start from SW, then you may be able to visit Isle of Scilly.
Generally 60-80miles/day is more than enough when you are with family.
We did Poole to Exmouth twice last year with a Rodman 28 and it was an amazing experience.