Many routes match that specification
A key question is how active you want to be. Do you want to be working locks and (on some canals) swing bridges?
Or would you prefer a contour canal, which has few locks but meanders round the countryside to stay level, remaining on a contour.
The Oxford Canal is a prime example of that. Lazy, hazy days in great countryside through beautiful little villages.
If you are happy to work locks do you want a narrow canal, or a broader one?
The Grand Union, from London to Birmingham, is a broader canal. In its day it was the I90 or the I5 of the waterways system. Built to handle lots of traffic 24 hours a day at speed. So the locks fill and empty fast, and the gates are meant to be handled fast, with built in short cuts to work them faster.
A really good crew can work the locks on there very fast. I've been on a pair working down the GU when we've set ourselves targets to keep ourselves fit and showcase our experience, and keep bettering previous "bests". Great fun. Equally you can take your time.
I was on the Shroppie (Shropshire Union) on Tuesday for the first time in years and had forgotten just how nice a canal that is.
Two that might suit you to a T are the Leeds and Liverpool, and the Cheshire Ring. People often cruise in rings, using multiple canals.
The Cheshire Ring is a marvellous example of that. It is invidious to divide out best bits of that ring, but to me the Peak Forest Canal is the very best of a superlatively good ring. I would say that, but mean it, as it passes through my home village.
PS- A pair is a motor boat and an unpowered butty boat behind. You can't self drive hire pairs.