Hi Bin, and welcome to CRT!
I agree with the previous posters. While C.R. is a small country (about the size of West Virginia), travel to the beaches can be very time-consuming.
However, since you asked:
My favorite beach town within a 4 hour drive from San Jose is Manuel Antonio. The Parque Nacional Manuel Antonio was C.R.'s first national park, and the beaches there are tremendous, particularly Playa (beach) #3 in the park.
There are a number of hotels in the area that permit gringo/hooker guests. I like Costa Verde, but there are numerous others.
The problem with finding a companion for the weekend on such short notice is that many of the women (if not most) have ch*ldren and family obligations and will not be able to do even an overnight on such short notice. Not to mention that it can be difficult to find a girl with the right combination of looks, price, personality etc. Speaking Spanish will be very useful in your search, so if you don't speak Spanish, better find a wingman to help you with the interviews.
Try this: Pampering? Stay at the Sportsmen's Lodge or one of the other nearby hotels (Amistad, Castillo, Dunn Inn, Vesuvio, Clarion Amon Plaza, Hotel Little Havana). In the case of the SL and HLH, there are girls on-site who will provide anything from an excellent non-sexual massage to a pronstar experience. You'll still get to eat and drink the local cuisine, and any of these hotels will know how to get you onto a fun day tour. Zoo Ave/Bird Zoo has been mentioned. There is also a three-in-one tour that gets you close to a volcano and includes a tour of a coffee plantation. You could spend the afternoon disturbing the ecology on an ATV or even make a run to Jaco, just to check it out.
I believe the absolute shortest trip on which you could accomplish your goals would be 5 days or more. This is especially true if theis is your first trip to C.R., your first trip which will include fun with the prostitutes, or both.
Read the Newbies section, Hotels section and the General Information section to learn a lot of useful stuff. Use the Search function to get specific questions answered. And be sure to let us know how your trip went!
Again, welcome to CRT. If you're planning any trip, a V.I.P. membership will get you a discount card that can save you the cost of membershipt in just a couple days. And, your V.I.P. membership also allows you to see the photos/vids in the galleries and participate in a members-only forum here on CostaRicaTicas. And no, I'm not an employee or owner of CRT; I'm just a Lifetime Member.
Good luck!