Short answer: Plymouth is cheap to do properly because the Hoe, the Barbican and the Box are all free, and Devil's Point gives three hours free parking. The aquarium is the one worth paying for, and its ticket includes free re-entry for a year, which makes it better value than it first looks.
Park at Devil's Point, which costs nothing for three hours, and walk the waterfront. Carry on to Royal William Yard and look round the converted naval buildings — there is no charge to be there.
Into the centre for the Box. Free, and the 4D film is the bit that justifies the walk. Allow a couple of hours.
The Hoe and the Barbican, both free. The Hoe gives you the view that explains the city; the Barbican gives you the streets. Boat trips leave from the Barbican if you want to spend something, and seeing the Sound from the water is the one paid extra worth considering.
The National Marine Aquarium is the UK's largest and the ticket includes free re-entry for a year. If you live within an hour of Plymouth that is not a day ticket, it is a season pass priced like a day ticket. Closed Wednesdays.
The Village Restaurant does a good-value set lunch and specialises in seafood — book, and know it gets loud. Fig & Thyme is the best brunch on the Barbican but you need to arrive early or wait. Pier One opens at 7am daily with a log burner and sea views, which makes breakfast there a cheaper way to use the waterfront than dinner on it.
Barbican parking is expensive and it is the default people choose. Between that and an evening meal on the waterfront you can spend fifty pounds before you have done anything. Devil's Point and a lunch rather than a dinner keeps the day inside the number.
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