Delta is offering up to 25,000 bonus miles for activity with their partners. Registration is required. The offer is 5000 miles for each 5 unique partners with activity during the qualifying period of April 1 – May 31 (too tight a timeline for my tastes).
Airline partners don’t count. Each partner counts only once, so only one Hilton stay would count as a partner (but a Hilton stay and a Marriott stay would be two partners). Transferring points into Delta should count, and each source should be a different partner, so I may transfer points in from Starwood, American Express Membership Rewards, and Radisson Goldpoints.
Read the terms and conditions on the promo, but it should be pretty easy to rack up 5000 bonus miles — much harder to hit a full 25,000.