I would try and replave the 1900 switch if your budget allows you to as the 1900 switch isn't tested anymore on the CCNA Exam.
As you have three routers you could practice setting up static routes and routing protocols such as RIP,EIGRP,OSPF. THe CCNA sybex book has labs which you can follow.
You could have your two 1600 Routers connected upto your frame-relay router and you could implement frame-relay. You could also connect your two routers together to implement access-lists and setup loopback interfaces to test that they work properly.
I would practice on your routers doing basic setup so you get used to the different commands etc.
Hope that helps.
