Try to meet locals. Local people will tell you about the best destinations. You can drive by rented car to beautiful landscapes and visit the most beautiful attractions, but there is no better way than when a local tells you an anecdote or gives advice about a place.
If you've come to a destination and see that you've been overspending on the trip, you can freely change the plan for the site and adjust the time and money.
But I digress as @adonisabril said it is crucial to plan ahead.