I dont think you understand.

The router has DHCP turned on.
In addition to IP addresses, it also gives out the router option to the client, so that their default gateway points to the router's IP.

I want to know if there are other options that can be set and handed out by the router to the DHCP client machine.

for example, option 66 for boot server

Apologies I understand – I checked the Telnet command reference just in case it was something hidden in CLI no joy I'm afraid.

"e.g. option 66 for tftp boot server?"

Sorry I dont understand what you are trying to do, what is option 66 for tftp boot server?

there is only one DHCP range on the Draytek unless you have external IPs and want to assign them via the second subnet of DHCP addresses

DHCP options is specified under the RFC-

http://www.networksorcery.com/enp/rfc/rfc2132.txt