Give your students this challenge, which is very similar to the mutilated chessboard puzzle. The mutilated chessboard problem is so popular that students may have already seen it, which is why I don't advise teaching it. If you know students haven't seen the mutilated chessboard problem, you are welcome to give that problem instead. It can be found here.
After giving either of those two problems, ask students if it's possible to fit all 35 free hexominoes into a rectangle. Give your students a drawing of the 35 free hexominoes, such as this one.
Next, give your students this challenge. Have your students solve the first part of the challenge before letting them use the interactives on the NRICH page. We want to let students discover the strategy of drawing boxes. If they don't come up with drawing boxes 1x1, 2xx2, and 3xxx3, draw 2xx2, then have them draw the others. After that, they should be able to rearrange mentally, or by making a sketch, to find at least one of the two solutions. I believe not using the interactive will put more emphasis on thinking about the problem and take a bit of emphasis off just dragging-and-dropping until a solution is found. After students have found both solutions to the first part of the problem, allow them to use the NRICH interactives to solve the remaining parts of the challenge. Then give your students this challenge. I recommend delivering it without showing the NRICH page. This way you can give the hints when students need them. Otherwise students may just reveal all hints without thinking. You can let students use the interactive from the NRICH page by following this link.