Suomenlinna is certainly a good option, bring a flash light and you can explore the dark corridors within fortress walls. Classic sights for kids in Helsinki are of course amusement park Linnanmäki and Korkeasaari zoo, for the latter there is possibility to take a boat from the Market Square. But I guess these are not that superspecial compared to other amusement parks or zoos elsewhere.
Now, if climbing interests you, Adventure park(s) Korkee might be good choice as it offers adventure courses on trees. There are two locations of these parks, Mustikkamaa might be more child friendly choice and it is located on the island next to Korkeasaari, so it could be possible to combine the two. Other location is at Paloheinä in northern Helsinki and there is only a treetop course that accepts customers that are 110 cm or longer. Sadly the website of Korkee appears to be only in Finnish, but the images there at least give some idea what sort of activity these parks offer.
If it is warm (or even hot) weather, I guess water activities would be perfect choice to spend some time. For more than just basic swimming next to a beach there are few waterparks in Helsinki region: Laguuni and especially its Keilaniemi (Keilis) venue in Espoo might offer most, although they have some activity also next to the beach at Hietaniemi (Hietsu). Another floating water park is located at Rastila beach in eastern Helsinki, sadly Lappis water parks has only Finnish language website at the moment. Then there is also the classic waterpark Serena in Espoo, but the location is somewhat remote. And Flamingo spa waterpark in Vantaa offers also water activities, but it is entirely indoor facility. If you don't necessarily want fancy water activities, I think one worthy contender is Swimming Stadium, which was the venue for 1952 Olympics swimming and diving contests and these days is open for the general public for a modest fee. And most (if not all) these places do have a sauna if you are looking for one.
To continue the theme of Olympics, the Helsinki Olympic Stadium might interest you, the stadium tower offers nice views over the city and there is also a sports museum. And as you mention soccer, I think it is worth mentioning that July 11th to 16th there is the big youth football tournament Helsinki Cup. It might not be possible to join the teams participating in that competition for one day, but at least it would be possible watch the matches on various football fields around the city. Friday and Saturday will be the days for the finals, which are going to be played in Töölö near the Olympic Stadium.
I guess the above is more than you can cram into a single day cruise visit, but I hope I gave some plausible alternatives to spend some time in Helsinki (and possibly neighbouring cities).
And regarding snacks, I think simple and fairly healthy options would be picking up some fresh fruits and/or vegetables from a market vendor. July is the prime season for strawberries in Finland and peas (sold in pods) are a popular choice. And if you pick anything to eat from the Market Square, be aware the local gulls have became quite talented in stealing food.