The AI opponent is basically weak. You can still have an interesting game by using handicapping. First you need to decide whether you are playing 1 country or entire alliance.

If you want to play 1 country, you will need to reduce your capabilities. Try the following.

• Withdraw after the first round of combat. This mitigates the AI's inability to consider counterattacks when attacking. It also gives the game a more natural feel.
• No amphibious invasions of enemy capitals (except for island capitals). The AI can't properly defend against these and withdrawal is impossible.
• No more than 1 bomber can strategically bomb a factory each turn. In some scenarios, strategic bombing is too effective and the AI does not respond properly to the attacks,
• Buy exactly 1 tech roll per turn (unless your income is 19 or less). This will increase variety.

If you are playing an entire alliance, you can use TripleA cheat options (starting with version 1.5). A 25% income bonus should make a reasonable challenge. Some prefer to add 1 to the AI's defense. You should still not invade enemy capitals amphibiously (except islands) because of the AI's limitations.

You could also use a fixed amount as a cheat, especially if you are facing only 1 foe or you foes are roughly equal in size. An attack or defense bonus throws off the balance between the unit types.

Bidding can also be used, but the AI tends to use large amounts of IPC's poorly.

If you are willing to change the game's XML files you can be flexible. Change the run count for bids from 1 to 999, and the bid will be received each turn. Delete the bidplace segment so the AI can't do anything with the money except spend it for regular builds (otherwise it can pop up new units in any controlled territory).