What are all the experience bonuses available?

Intelligence

Whenever you earn experience, the amount of experience you earn will increase, with higher levels of intelligence. Intelligence level provided through temporary buffs will also go towards your experience bonus. You can increase your intelligence in many ways:

• Favoring high intelligence over other stats, during the character creation phase.
• Picking up the "Your Special" book gives you an additional point to invest in your S.P.E.C.I.A.L. stats, which you can opt to add to intelligence.
• Collecting the intelligence bobblehead, although it would be wise to wait until you already have 10 intelligence, as collecting the bobblehead is the only known way to permanently increase your intelligence past 10.
• Various mods and legendary bonuses may infer intelligence boosts while equipped.
• The Night Person perk increases your intelligence at night time.
• Eating Mentats will temporarily increase your intelligence.

Also note that higher levels of intelligence also decreases the chance of receiving an Idiot Savant bonus. Depending on your play-style, you may wish to favor lower Intelligence, in order to further benefit from Idiot Savant.

Idiot Savant Perk

The Idiot Savant perk provides a chance of multiplying experience whenever you earn it. Rank 1 requires 5 luck, and infers a 3x bonus. At level 11, you can invest in Rank 2, which increases the bonus to 5x. At level 34, you can invest in Rank 3. While the last rank does not increase your bonus, it gives you the chance of receiving an additional temporary buff, that gives you an automatic 3x bonus to experience earned through kills.

This perk has a higher chance of activating with lower levels of intelligence. However, if you know you are about to receive experience you can simply save your game, and repeatedly reload your save until the perk activates and increases your experience gain. This is especially effective prior to handing in quests.

Sleeping Bonuses

Sleeping in an owned bed confers a temporary sleeping bonus, which lasts 8 in-game hours. Sleeping by yourself gives you the Well Rested temporary perk, for a 10% increase in all earned experience. Sleeping with a romantic partner gives you the Lover's Embrace temporary perk, which gives you a 15% increase to all earned experience.

The sleeping bonuses are not stackable, but there is a known bug, where the player might be able to receive both temporary perks at the same time, in rare situations.

When the player fast travels, in-game time moves ahead by the approximate travel time. This means that traveling outside of the remaining time will remove the temporary perk. PC users can reduce the timescale, using the console command set timescale to <m>, where m = game minutes to pass for every minute of play, which effects the buff timer and quick-travel times. set timescale to 2 will set the time scale to half its normal time, while set timescale to 1 will set it to its default.