Without downloading tools from "hacker" websites, you could easily simulate a simple DoS by generating a "Large ICMP Traffic" signature with an ICMP Echo Request packet with a size > 1024 bytes to a host on the "inside" of the router. Use a packet sniffer to demonstrate that the packet never reaches the inside host.
The following link documents IOS IDS Signatures that are supported:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fsecur_c/ftrafwl/scfids.htm#wp1000985
This document describes the TCP Intercept feature:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fsecur_c/ftrafwl/scfdenl.htm
You could use "Nmap" (or the Windows port) to generate a TCP SYN scan to simulate an attack.
You could also easily simulate some of the SMTP-related attacks by telnetting to an SMTP server on the "inside" and issuing Sendmail attacks, such as "expn" or "vrfy", and see that the connection gets dropped (assuming your router is configured to reset on attacks).
