Techniquest is great, but really busy on the weekend.
The swimming pool is good, but again really busy on the weekend.
There’s a Doctor Who exhibition in the Redragon Centre if your kids are into that.
There’s always free shows kicking off at the Welsh Millenium Centre down at the bay if your kids are into the arts. whilst you’re there you can visit the Senedd (National Assembly for Wales) and hire a rickshaw and visit the barrage (pretty impressive); on route there is a really cool park, bmx street area and outdoor gym with a climbing wall and footie nets(that kids love).
Not sure if Boulders and the Climbing station (Treforest) do ‘drop in’s’ but if they do that is a great way to kill a couple of hours.
Jamie’s for lunch; definately worth a small wait and a very slight price hike over alternatives. About 100 yards from the Lego store.
Hope that helps.