MyI would have to go alone on anything I'd like to go on and I wouldn't have any fun doing that lol. lol
If you should find that anyone in your party is not using up their 3 FP on any given day in any of the 4 parks, then you absolutely SHOULD go on them by yourself!
With a FP, it will only take you about 15minutes tops to go on an attraction. AND- truly- at WDW your spouse will absolutely not be stuck with nothing to do, unless he REALLY chooses to sit and do nothing. Even then, most of us enjoy a little people watching now and then! WDW also has a number of playgrounds and things for children that don't want to ride.
All of the Future World pavilions have walk through areas that aren't part of the 'ride'. The big Disney joke is that all rides end in a gift shop. Not quite, but there are themed gift shops for many. There's also many more activities.
Let's talk Test Track- as one specific example for a moment. The post ride exhibit is fairly extensive. There are multiple activities for your DF and daughter. There are multiple spots where they can pose for photos in virtual surroundings. There's a real car showroom- with the latest Chevy models to explore. There is also a virtual racer car racing activity. It is rather like the old slot car racers of yore (or being able to race virtual electric toy trains).
Further, If we ALSO went though the pre-ride area with you- then each of you would actually be able to design your own virtual cars for that activity. The three of you would only be apart for just the durartion of the actual ride - approximately five minutes. If you use a Fastpass, and he opts again doing the pre-ride for TT, then you'd still only be apart maybe 15minutes tops.
BUT...you also don't even have to waste a FP for you to ride TT, because there is ALSO a Single rider line, just for solo riders. The wait for that queue is variable, but early in the day it is often under 20minutes (total ride time).
Truly though, many WDW rides are on par with riding in a slow moving car.
The Seas' Nemo ride is very tame...but if DF really doesn't want to ride, then there are extensive things to see post ride. Giant aquariums big enough for sharks, dolphins, and manatees. Smaller aquariums with all kind of critters, and electronic quizzes.
He also has options through his doctor if his motion sickness is bad enough that he can't tolerate a merry go round, if he might be willing to explore those options. There are even things like pressure point bracelets which can help. (that aren't internal meds). Those might easily provide enough relief for him to enjoy the many tame rides WDw offers.