From the Citadel look over at the Governers House. Walk past that and cross the river and you'll see a huge park on the right. And, from there, if you walk around the lake back to town, you will find a place where you can rent kayaks and either do the whitewater section of the river, or the other fork where it's smooth. And just past that is Luxembourg Square which is another large park with a really interesting church tower.
Or you can just turn to the right, walk around the edge of the Citadel, and you find a park around the small chapel, and further along the fort wall the walled in herb garden.
You can walk along the river all the way down to the Devils Gate where the Seille joins the Mosel.
Don't forget to visit the indoor market, right by the main cathedral, or the Cour de l'Or Museum.
If you're there over a weekend check and see if there's a flea market. Google "Metz Puces".
Trier is an easy visit, as is Nancy with long lists of things to see and do in both. As Fred mentions there's a lot of interesting little towns also. Plus there's the huge American cemetery at St Avold, and Maginot Line forts close by. Verdun is fairly close, but if you think about going there let me know and I'll clue you in on a number of other monuments you can visit on the same trip if you have a car.
And then, if you're interested in iron or coal mining there's a couple huge mining museums with underground tours about 40 minutes from Metz.
You want to walk around the Pont Moyen in the evening and get a photo of the Temple Neuf when it's all lit up.