Hotel Nacional is well worth just the one night at least. The breakfast is amazing (all world cuisines covered in one room). Mojitos in the garden with bands serenading and the sea view in front - it's an amazing way to start (or end your visit).
A Casa particular in Vedado (Habana). We stayed with Horacio and his wife, and were made to feel very welcome. He speaks fluent English, is extremely enthusiastic about his city - making recommendations, made good (but simple) breakfasts, and as we were three (one couple) travelling, the fact that there were two rooms to hire was ideal.
However, the outstanding feature was the house itself, with (as the name suggests) antiques abounding, a lovely balcony, grand piano and huge four poster beds. We were stuck in the house during Hurricane Charley (2004) and despite this, it was such a memorable end to our month in Cuba. Highly Recommended.
The Santiago Festival (end of July) is a great experience, but it doesn't get started until 10/11pm, and continues well into the early hours. It involves many lavish parades with dancing and costumes, and there is a great party atmosphere. The only problem is that you need to get there early ( 8pm) to get a good seat.
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