Train or bus from Göteborg, Malmö or Borås. For those with a lot of energy Varberg is also easily reachable by bicycle from both north (Göteborg or Kungsbacka) and south (Falkenberg...) through nice bicyle paths.
The city center is easily accesible by foot. Other options are bicycle and car. There is a public transportation net but not very good.
Get a spa at apelviken or just walk around... Varberg also has some of the best wind surfing in Sweden.
Varberg has several different restaurants and bars serving food. It also has a fish shop in the harbour for those wanting to make their own meal.
Varberg has several bars and a good nightlife in the summer. Most bars are open to 2am on the weekends.