In each scenario, there's usually an objective you have to complete. These goals can vary from having a certain number of guests in the park or building a certain numer of roller coasters. This page will give you all the possible objecives you'll encounter from RCT to RCT 3.
Roller Coaster Tycoon
"To have at least X guests in your park at the end of October year X with a park rating of at least 600."
The most common of the objectives, first introduced in Forest Frontiers. You can bring guests in your park in many ways, like build new rides for example. As for the park rating, you need to keep your park in good shape. Once you've gotten the required number of guests and park rating, all you can do now is wait until the required number of years has passed. If there are not enough guests in the park or the rating has dropped below 600 when the number of years is passed, the objective is failed.
"To achieve a Park Value of at least $X at the end of October year X."
A less common objective introduced in Diamond Heights. You can increase your park value by building rides, but their value will deteriorate over time and you'll soon have to replace them. This will go on until the required number of years have passed.
A special objective that appears in Mega Park. As the objective says, you have the freedom to do whatever you want with the park. A park with this objective can neither be won or lost.
"To have 10 different roller coasters operating in your park, each with an excitement value of at least 6.00."
Introduced in Coaster Crazy, the main goal here is to build 10 different roller coasters with an excitement rating of at least 6.00. In order to succeed, you'll need to use the terrain, scenery and your own rides to your advantage and build when you have the budget for it. It is not possible to lose a park with this objective and there is no time limit given.
"To finish building all the partially built roller coasters in this park, designing them to achieve excitement ratings of at least X."
Starting in Volcania, you may get this kind of objective. You'll start out with five incomplete roller coasters and your goal is to complete them with a certain excitement rating. Much like the objective above, you'll need to rely on your surrounding in order to get your desired excitement rating. While the original RCT gave you a limited budget for these scenarios, in RCT2, you could only get this type of objective if it's a no money scenario. Like above, losing isn't possible.
"To have at least X guests in your park. You must not let the park ratingdrop below 700 at any time!"
First appears in Arid Heights, this is similar to the first objective, except you don't have a time limit. Instead you can't let the park rating drop below 700, if you do you'll get a warning telling you that you need to raise the park rating within 4 weeks or the park will close (which mean that you've failed the park). It can not be re-opened since the park open/close buttons aren't visible. The objective originally only appeared in a no money scenario, but in RCT2 you can have this objective in a regular money park. The flag button to open/close the park will be grayed out then. A park with this objective can even be lost if it already has been won before!
"To have a monthly income from Ride Tickets sales of $X per month."
Introduced in Vertigo Views. In these scenarios, you'll have to build a lot of rides in your park (mainly roller coasters), but remember that you will have to decrease the ride prices as the ride gets older. Your ride tickets has to be the amount stated for at least four months to win.
"To have 10 Different types of roller coasters operating in your park, each with a minimum length of X ft/m2 and an excitement value of at least 7.00."
It's similar to the 10 different roller coaster objective, except you not only have to have a higher excitement rating, but you also need to make the rides a specific length as well. Introduced in Octagon Park, you need to use your surronding for an exciting roller coaster, but it'll probably take you many in game years to get the money to build all these rides in the original RCT. The only scenario with this objective in RCT2 is Ghost Town, where the minimum length is 3,937 feet (1,200 meters).
Roller Coaster Tycoon 2
"To repay your loan and achieve a Park Value of at least $X."
It's similar to the park value objective except you need to repay your loan as well. Try to find ways to bring in lots of cash and use the money to build rides and repay your loan. This objective starts appearing in Gravity Gardens.
"To achieve a monthly profit from food, drink and merchanidise sales of at least $X."
You'll start to see this objective in Bumbly Bazaar, this is similar to the monthly ride tickets objective, but this time you're dealing with shops. Build shops with a good amount of profit when the time comes.
Roller Coaster Tycoon 3
Roller Coaster Tycoon 3 features a different selection of objectives, due to the fact that you can fast forward and complete the objectives at any time. Additonaly, you have three to six objectives per scenario, classified by three different difficulty levels: Apprentice, Entrepreneur, and Tycoon.