Cheats:
At the main menu screen of the demo enter the "Game Options" and Enable the Console. From there, push your tilde (~) button to bring down the console and type the following:

seta thereisacow "1337" followed by spdevmap eldaba

This will start the demo level with devmap and cheats enabled. This allows you to use any of the commands in game without restriction. When in game, bring down the console and enter any of the commands from below to activate the 'cheat'.

jump_height - 128 Moon jump (jump_height 39 for default).
give ammo - Refill your ammunition / grenades.
timescale .25 - Slow motion (timescale 1 for default).
notarget - Enemies ignore you.
noclip - Walk / Fly through walls.
god - Infinite health.
give all - Each time it's entered you'll be given 2 different weapons from the demo.

Play Any Mission:
Bring down the console in game with your tilde key (the key to the left of "1") and enter the following to play your desired mission. This will allow you to play missions you are yet to unlock.

map moscow - Play "Red Army Training"
map demolition - Play "Demolition"
map decoytrenches - Play "The Diversionary Raid"
map decoytown - Play "Holding The Line"
map elalamein - Play "El Alamein"
map eldaba - Play "The End Of The Beginning"
map tankhunt - Play "Repairing The Wire"
map trainyard - Play "Railroad Station No. 1"
map downtown_assault - Play "Downtown Assault"
map cityhall - Play "Stalingrad City Hall"
map downtown_sniper - Play "Comrade Sniper"
map libya - Play "Crusader Charge"
map 88ridge - Play "88 Ridge"
map toujane_ride - Play "Armoured Car Escape"
map toujane - Play "Retaking Toujane"
map matmata - Play "Assault On Matmata"
map duhoc_assault - Play "The Battle Of Pointe Du Hoc"
map duhoc_defend - Play "Defending The Pointe"
map silotown - Play "The Silo"
map beltot - Play "Prisoners Of War"
map crossroads - Play "The Crossroads"
map hill400_defend - Play "The Battle For Hill 400"
map hill400_assault - Play "Rangers Lead The Way"
map bergstein - Play "Bergstein"
map breakout - Play "The Brigade Box"
map newvillers - Play "The Tiger"
map rhine - Play "The Crossing Point"

Unlock Any Mission:
The following cheats will allow you to unlock any mission of your choosing in the mission select screen. Bring down the console with the tilde key (button to the left of "1") in game and type the following commands to unlock your desired mission sets.

/seta mis_01 9 - Unlock "The Winter War" Mission Set
/seta mis_02 9 - Unlock "The Battle Of El Alamein" Mission Set
/seta mis_03 9 - Unlock "Not One Step Backwards!" Mission Set
/seta mis_04 9 - Unlock "Fortress Stalingrad" Mission Set
/seta mis_05 9 - Unlock "The Tank Squadrons" Mission Set
/seta mis_06 9 - Unlock "Rommel's Last Stand" Mission Set
/seta mis_07 9 - Unlock "D-Day" Mission Set
/seta mis_08 9 - Unlock "The Battle For Caen" Mission Set
/seta mis_09 9 - Unlock "Hill 400" Mission Set
/seta mis_10 9 - Unlock "Crossing The Rhine" Mission Set

Command to Show FPS:
Shows your Frames Per Second in the game. Useful for benchmarks and likewise.

/cg_drawFPS 1 - Simple FPS Display
/cg_drawFPS 2 - Verbose FPS Display
/cg_drawFPS 3 - Time FPS Display
/cg_drawFPS 0 - Display Off