So instead of making sweet love to the Misses under a star lit night sky, you're going to be playing the PSP instead?
LOL, just thinking of my younger days..
I'd suggest Starwars Battlefront II for the "Instant Action" mode, real fun in quick bursts if you can get used to the controls. (The lack of a second analog stick really kills FPS's on the PSP)
If you've played the PS2/Xbox version you pretty much know what to expect, except instead of 16 bots per team it's watered down to 10 bots each team.
It's a greatest hits now though so it's only $20.
GTA Vice City Stories. Since it's so open-ended you can pretty play for as long as and save whenever you want. Most missions only last about 15 minutes, and you don't even have to play the missions.. if you just feel like going on a killing spree in town and seeing how long you can outrun the cops, you can of course do that too.
I'm assuming you meant Dungeon Siege also? Heard nothing but good stuff about that came (I have 2 copies that are still sealed, will get around to eventually) plus it's only $9.99 at Best Buy if you can still find it.
A few other games that have bots (so you dont even have to play through a campaign) are 50 Cent Bulletproof, Eragon, From Russia with Love, and Pirates of the Carribean.
And if you ever take your PSP online at home then for the love of god GET SOCOM! I think I'm the only person on this board who even plays Socom online.. (I have both 1 & 2 and play them equally)