This is the pre-installed scenario that comes with RCT2 that resembles a sandbox the most (although you can obviously use the Scenario Editor to create something that more closely resembles a proper sandbox). With a starting loan of $10,000 that can be increased all the way up to a whopping $100,000, the huge loan ceiling should be sufficient for you to build almost any ride you want. Getting 1,500 guests in your park at the end of Year 4 may seem challenging (especially if you felt that Crazy Castle's objective was difficult even though it is a pay per ride scenario), but this time limit should be sufficient as long as you're constantly building new rides.
Unfortunately, this is also a park entrance fee scenario, so the rides are free. To mitigate this, you'll need to find some way of ensuring that guests continuously step through the park entrance gates. The relevant section in Hints and Tips contains some advice for scenarios like this one, so consider giving that a read as well.
Begin by opening the rides list, going to the Research tab and unchecking Shops & Stalls. If you feel a need to get all the available rides that can be researched first, feel free to uncheck Scenery/Themeing as well. Next, build a ride or two—a Merry-Go-Round and small roller coaster is a good starting point—before opening the park.
The lowest amount of cash a guest will start with in this scenario is $60, so increase your park entrance fee to this amount when you open your park.
There is a huge variety of rides available without having anything researched yet—you even get all of the available transport rides from the get-go. One thing to keep in mind, however: start small. You may feel tempted to blow a large chunk of your loan ceiling building a few expensive roller coasters (like the larger Six Flags coasters such as Texas Giant), but this may work against you in the early-game as the number of guests entering your park is determined largely by the number of rides currently operating in your park. In addition, while the loan interest is only 5%, this amount can add up quite fast once you are halfway up the loan ceiling—5% of $50,000 is $1,000, for instance—which will more than likely eat through whatever revenue you can make at the park entrance gates within Year 1. A more prudent, tried and tested strategy is to build a decent number of rides that are flat/compact and cheap first and let them increase the number of guests entering your park before building a large coaster design. These rides will keep the guests streaming into the park at a steady rate, giving you a constant flow of income for a while, which will go a long way towards the construction of your first large coaster or equivalent. Once you start generating a regular income of about $1,500-2,500 from Park Entrance Fees alone, you can get a bit more ambitious and build larger, costlier rides without worrying too much about the loan interest.
It may also be worth your while, given the huge size of the park, to think about "decorating" the areas where you are going to build, or have already built, some rides. A majority of the common Scenery/Themeing objects are given to you at the start, so feel free to experiment with dividing the park up into little themed zones. One of the nice things about setting up a constant flow of income is that you can sink funds into landscaping projects, instead of sinking funds into building larger rides, to make your park look more themed and "authentic". The icing on the cake for doing so would be getting the "Most Beautiful Park" award, which draws guests to your park on its own for about two months without you needing to build any new rides.
Beyond these pointers, the strategy to beat this scenario is simple: keep building rides to get the guests (and cash) coming in at a regular rate, and keep an eye on your income versus your expenditure.