They have several museums, and I think you can buy some sort of pass that will include a number of them. Some are old mansions, there's the diamond museum, the lace museum, and plenty others.
You can climb up the bell tower (300 steps), check out the church.
Walk through the streets and open marketplace. There will likely be some great hot foods you can pick up as you walk, and there will be plenty to see.
There's a bus tour of the Old City. Of course, if you're a walker, wander the streets. It's a quaint, lovely place to see.
Of course, there's also shopping. Loads of little shops, restaurants, great little places to step into. about 4 years ago