Unfortunately, there isn't. The entrance fee to Sentosa Island ranges from $2 to $6 per entry, depending on the day and time. The cheapest time slot is during lunch (12nn to 2pm) and dinner (5pm to 7pm), when it's only $2 per entry. Otherwise, it's pricey at $6 per entry.
However, if you're a frequent visitor to Sentosa. I'd recommend getting the Sentosa Islander Membership which entitles you FREE entry into Sentosa via the Sentosa Boardwalk, Sentosa Express and Sentosa Gantry (drive-in). The annual pass costs $25 per year and assuming you drive there on the weekends when the entrance fee is $6, you only need to drive to Sentosa more than 4 times a year to make your money's worth. After your first year, renewal is only $20. The individual membership is non-transferrable though, and only entitles one car and one person to the entrance fee waivers.
If you’re a dad or mum, get the family membership ($50) instead. You’ll be the main cardholder and your children/relatives will be the supplementary members. (up to 4 pax, immediate family members only).