Okay, cool that if you have GMRS. This is the part that will help you a lot.
Check your manual and see which channels use FRS and which use GMRS. If there are channels that's pure GMRS, use those channels exclusively for better coverage.
Then, NEVER choose a channel without choosing a sub-channels. Most people don't know how to access the sub-channels making most people sharing the same channels.
Therefore, say the GMRS channels are 16 - 24, choose something like channel 18, then from there change the subchannel to 13.
Believe it or not, in any channels, statistically people won't use sub-channel 13 because of the superstition of being bad-luck number. The clearest FRS channel is usually channel 13, sub channel 13.
Regardless, now that you use GMRS, something like channel 18 sub-chanel 13 will virtually be a sure bet that nobody will use that.
The lowest end GMRS product will easily give you a 2-mile range in open space with no interference (forget the 5-mile theoretical limit, that will never happen).
In the very worst case scenario, say if WDW is filled with EMI and RFI, you will have at least 1-mile range from GMRS.
PS: I used to own an FRS radio (also Cobra with the total of 256 subchannels), and when I used it in a heavy EMI/RFI area, the range became only about a couple hundred yards.
Hope it helps