Dear Chorlton_lady,
As you guessed this question gets asked a lot. Here's a few thoughts that I've cut and pasted many times...
=====================
The "best way" to see Havana is of course entirely up to you and your comfort level off the resort, as well as your budget and whether you can afford to stay over for a night or two.
1.) On one hand you can buy a package tour from your resort. It's simple and hassle free. You get picked up from your front door, taken to Havana and your guide will shepherd your tour group through every step of the way. You are just one of many sheep in the herd though...
2.) On the other hand you can jump on a Cuban bus, go to Havana, do a self guided tour, then catch the bus back to Varadero.
Between those two extremes there are MANY different options and choices. It's up to you to decide how you'd like to handle it.
======================
My personal recommendation for a first time visitor who doesn't speak much Spanish would be something like this...
Take the early Viazul bus to Havana and be met by your local guide. Check into your Casa Particular then enjoy a one-on-one tour of Old Havana for a few hours. Have a nice lunch then do a few things that you've asked your guide to arrange in advance for you... maybe it's a visit to a cigar factory, do a ride down the Malecon in a classic convertible, check out the Colon Cemetery, view a Santeria ceremony, do a dance class or visit an athletic facility like a boxing arena. Go back to your casa to freshen up and maybe have a nap, then have a meal at a great paladar (private restaurant) before catching a show at the Gran Teatro (Grand Theatre) then go out and paint the town red.
The next day have a great breakfast and one last walk around Old Havana before splurging on a taxi directly back to your resort.
======================
Transport between Varadero and Havana:
1.) Taxi: Fast, efficient and expensive. Official cost is 90 CUC one way. Travel time about 1.5 hours and they'll take you right to your doorstep.
2.) Viazul Bus: Cost is 10 CUC one way and takes about 3 hours. It stops once at the end of the Prado, beside the Malecon on the edge of Old Havana (this is where you would hop out) a second time near the University and lastly at the Bus Station beside the Zoo in Vedado.
http://www.viazul.cu/asp/reserva/ruta.aspx?id=20
3.) Taxi from Viazul Bus Station: When you get to the bus station you'll be approached by someone wanting to sell you a seat in a taxi instead. Don't be intimidated, this is a legit enterprise with taxis competing with the bus. They'll offer you a seat for a few more CUCs than the bus costs, but the advantage is that it'll be way faster and they'll take you directly to your destination instead of simply being dropped at the bus station.
A few points to remember... Tell the driver where you're going and confirm that the price includes a drop-off right at your address. Take a look at the taxi and see who else is going... you don't want to be stuck in the back of a broken down Lada between two big guys carrying chickens. Generally though the taxis offer a great alternative service, so check them out before you go inside the station and confirm a bus seat.
4.) Transfer Colectivo: Most tour agencies will run shuttles between some of the Varadero resorts and some of the Havana hotels. It's more expensive than the Viazul Bus but it does pick you right up at your resort, thus saving taxi fare.
Note: Transfer Colectivo schedules/prices are open to change, it also depends where you are located in Varadero - you might be stuck on the bus for a long time as it goes from resort to resort picking up other people.
======================
1.) The best money you can spend before going to Havana is to buy a Guide Book like Moon, Rough Guide, Lonely Planet, etc.
2.) The best money you can spend after arriving in Havana is to hire a local guide, at least for part of the first day until you're acclimatized. There are lots of excellent guides available, but very few of them have an Internet presence. Do a search for "Jorge" or "Pototo" here for two excellent choices.
3.) Everything you need to know about Money Exchange:
http://debbiesreviews.proboards.com/index.cgi?board=cubageneral&action=display&thread=18675
4.) Steve's great self-guided Havana tour:
http://www.7daysinparadise.com/smf/index.php?topic=6052.0
5.) Tropicana: If you enjoy and appreciate Las Vegas style cabaret entertainment with a live orchestra, dozens of dancers and entertainers then the Tropicana won't disappoint. It's the same style entertainment that's offered at your resort x 1,000. Tickets are 70 CUC, 80 CUC and 90 CUC plus the taxi back and forth of course. Any tourist desk in any hotel can arrange this for you.
6.) The Gran Teatro - or for that matter, almost ANY venue in Havana - is (in my opinion) indescribably more "authentic" than the Tropicana which is sold on all the resort excursions. A couple of months ago I saw world class performances of dance, opera, ballet and several music concerts for less money than 1 ticket at the Tropicana. Here's a few ideas regarding arts/culture/music/etc. in Havana:
http://www.facebook.com/pages/Havana-Good-Time/139270972779729?ref=search
http://wonderfulhavana.com/whats-on
http://www.canalcubano.com/ingles/musica/centro.asp
http://www.cubaabsolutely.com/
http://www.havananightlife.com/
7.) One of my favourite things to do on a Sunday:
http://www.havana-cultura.com/INT/EN/cultura.html#/687
8.) Additional Havana info:
http://www.cuba-junky.com/havana/havana-city.htm
9.) Some great hotels in Old Havana:
http://www.habaguanexhotels.com/
10.) Some Casa Particulars (Cuban version of a Bed & Breakfast) in Havana:
http://www.cuba-junky.com/havana/havana-casa-particulares-home.htm
Have fun.
Cheers,
Terry