There are two credit cards with large sign-up bonuses that are set to expire this week.

The CitiBusiness/AAdvantage Platinum Select World MasterCard: 70,000 bonus American Airlines miles for spending $4,000 on the new card within three months of account opening. You must apply by today (October 9, 2018) to qualify for this bonus offer. 
The Citi ThankYou Premier: 60,000 bonus ThankYou Points for spending $4,000 on the new card within three months of account opening.

As of today, September 13, Citi has improved its transfer rates between their reward currency, ThankYou Points, and travel partner JetBlue TrueBlue.

ThankYou Points earned via the Prestige and Premier cards will transfer 1,000 to 1,000 TrueBlue points instead of the prior 1,000 ThankYou Points to 800 JetBlue points.
ThankYou Points earned via the ThankYou Preferred card will transfer 1,000 to 800 TrueBlue points instead of the prior 1,000 to 500 TrueBlue points.

I think we can thank the addition of JetBlue to Chase Ultimate Rewards' line up of travel transfer partners (at 1:1

There are just two days left to take advantage of the 30% transfer bonus between Citi ThankYou Points and Flying Blue, and three days left to book Flying Blue's August Promo Awards...the cheapest of which will get you from New York City to Paris for only 8,000 ThankYou Points if you stack both promotions.

The Citi ThankYou Premier Card is now offering 60,000 bonus ThankYou Points after $4,000 in purchases within the first 3 months of account opening. For the majority of 2016 the ThankYou Premier had no sign up bonus whatsoever, then Citi came back with 40k points for a $3k minimum spending requirement bonus in December 2016. Since then we’ve seen the bonus fluctuate between non-existent to typically no higher than 50k. 60,000 ThankYou Points is the biggest bonus the card has ever offered.

The Citi ThankYou Premier Card is now offering 50,000 bonus ThankYou Points after $4,000 in purchases within the first 3 months of account opening. For the majority of 2016 the ThankYou Premier had no sign up bonus whatsoever, then Citi came back with 40k points for a $3k minimum spending requirement bonus in December 2016.

Transfers of Citi ThankYou Points to JetBlue TrueBlue points will have a 25% bonus through 11:59 PM ET October 25, 2017.

This could be a good deal if you have an immediate use for TrueBlue points.

You can now transfer ThankYou Points earned by your Citi Prestige, ThankYou Premier, ThankYou Preferred, Citi Chairman, or Citi Forward card to Turkish Airlines Miles & Smiles frequent flyer program at a rate of 1:1.

That means ThankYou Points now transfer to 14 airlines and one hotel loyalty program.

The only partners that don't normally have a 1:1 transfer rate are Hilton (1:1.5) and Jetblue (1:.5).

The Citi ThankYou Premier Card currently has an offer of 50,000 bonus ThankYou Points after $4,000 in purchases within the first three months of account opening.

According to One Mile at a Time, that sign up bonus is changing tomorrow, August 22. This is not shocking considering that, for the majority of 2016, the ThankYou Premier had no sign up bonus whatsoever. Then in December of 2016, Citi came back with 40k points for a $3k minimum spending requirement bonus.

This coming Sunday, July 23, the Citi Prestige Card will lose some benefits and gain some new ones.

This is the current offer, as of today:

Sign Up Bonus: 50,000 ThankYou Points after $4,000 in spending within 3 months of opening card
Category Bonuses: 3x on air travel and hotels, 2x on restaurants and entertainment
Air Travel Credit:  $250 statement credit per calendar year ($500 during first year of cardmembership)
Global Entry: $100 application fee credit
Lounge access: Priority Pass lounges worldwide for you and two guests
Free golf: Three rounds of golf annually booked through Citi
Free