What is the easiest and cheapest way to get around towns and cities?

So I'm having my parents visit me from overseas in a few months and I wanna plan a nice vacation showing them around the country over the course of 2-3 weeks. How should I go about travelling between towns and cities(suggestions are welcome) for short day trips that won't break the bank and would also not be too tiring of a journey since my parents are a bit old. We would likely be traveling from either Birmingham or London around the country. Thanks in advance