At the National Aquarium in Baltimore, I work in our Living Seashore exhibit, which is a touch exhibit. In our main 3,600 gallon touchpool, we have animals from the Atlantic, including Atlantic stingrays, little skates, clearnose skates, horseshoe crabs and whelks. A 75 gallon tank is going to be tough to stock. You want larger, slower-moving animals that can withstand being touched. 75 gallons isn't a whole lot of room for that. Additionally, larger and slower animals are going to tend to be messy eaters, which means biological filtration is going to be difficult.
If you do have to make this a touch tank, I think horseshoe crabs are still excellent choices. They're slow and really interesting-looking. They survive in a wide range of temperatures and salinity, so they are really tolerant of different water parameters. As long as you don't have any rock work they can topple over, they're practically indestructible (they're 450 million years old). We feed them regular shrimp and mackerel from a seafood supplier. Just stay up on the water changes and nutrient control. We noticed that we had noticeable ammonia spikes in our touchpools on days where we had high volumes of visitors because of how filthy people's hands are. In 75 gallons of water, things can get really dirty really quickly.
