HopLop in general is a very good place to take your kids when the weather sucks. The Lanterna Hoplop itself is the best of them all.
It's a warehouse-sized large space, maybe 400 square meters in total, made of communicating cubes where kids travel, climb, hop, slide, bounce, fall through and swing by to get from one place to the other. There's rooms full of balls to swim in, elastic floors that you drop through, a huge toboggan where you can slide down on a tube, trampolins to bounce on, bumpy things, slippery things, foamy things and climby things! And this is not one floor; these cube rooms are stacked 4-high so we're talking around 1200+ cubic meters of fun and discovery all in one spot
And there's a "volcano" you can climb up and then slide down the middle (fun for climbers of all ages).
One of the most fun things you can do as an adult in HopLop is go look for your kid and get lost in there yourself.
The cost is high but the place in this case is worth it. You pay €15 for each child, though parents don't pay. There's a café with a restaurant that caters to small people's tastes, though they do have coffee and refreshments and well, the food is Finland healthier food, even if it's for kids.
They also have several private rooms that you can rent for parties: a princess room (yikes), a pirate room, and a disco room. You can book the room and special food and items for the kids. The room rental includes free use of Hoplop and every kid gets a discount coupon for another visit.
There's a decent parking lot right outside and it's free.