Muiden is a very small, quite picturesque old town, just South-East of Amsterdam.
Its major attraction is the Muiderslot (a medieval castle), that is worth seeing and visiting.
Other than that, Muiden does not have many attractions. Near the castle are some nice cafes and restaurants, where you can also sit outside and watch the boats go by.
Muiden harbour, Muiderslot castle
Muiderslot (in Muiden)