You can also buy land to increase your park value as well.
I also have a sneaky suspicion that the condition and age of the rides are also taken into account when calculating the park value. And it's possible that park rating is also factored in (although that might be me being paranoid).
If you have a "stable" and profit-generating park then simply leave the time on fast-forward for a few years. As long as you have a decent maintenance crew and have set the ride inspections to 10 minutes you'll get there in the end.