Short answer: The Eden Project. The Biomes are enclosed and heated, so rain is genuinely irrelevant — and your ticket lasts a year, so a wet day is a sensible time to use it.
St Austell has the single best wet-weather attraction in Cornwall on its doorstep, which makes the plan straightforward.
The Rainforest and Mediterranean Biomes are enclosed and heated. The Rainforest Biome in particular is warm and humid regardless of what's happening outside. Tickets convert to a year's pass, so a rainy day is a good use of one.
The china clay museum, with indoor galleries covering the industry that built this town. Genuinely interesting and much quieter than Eden.
The harbour is exposed, but the Shipwreck Centre gives you somewhere indoors, and the pubs around the harbour are good places to watch weather come in.
Indoors, with a food hall offering a genuine range — but only open Wednesday, Saturday and Sunday, so check before setting off.
Springtide at Charlestown has a log burner and is dog-friendly. The Boathouse is light, airy and does good seafood sharers.
Everyone else has the same idea, so the Biomes get busy. Arriving at opening or late afternoon is noticeably calmer, and the Rainforest Biome in particular is more pleasant without the crowds in that heat.
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