The responsibility of having a fish may be a bit too much for younger kids, so why not use that aquarium stored away in the basement to grow a mini-garden.
The kids will love it and take pride in knowing that they can create a mini-world that grows and changes day-to-day. The folks over at Canadian Gardening do it best, so here are some links to their how-to website including a great video to get you started.
How-to Video: Create your own Terrarium!
Canadian Gardening also have a great kids section on gardening. Check it out here to get the kids excited to start digging in the garden!