A couple of thoughts.

1) Have you already used your CSR travel credit? If not you can buy cash (looks to be $575 for UA) and use that credit perhaps combined with using points at 1.5 CPP to offset the rest.

2) This is the absolute best option I can find on UA (assuming you don't have status or UA CC to get access to XN space):

IAH - CLE - LGA
Departs IAH at 7:15am, Arrives 3:17pm (via CLE)

EWR - DCA - IAH
Departs IAH at 3:10pm, Arrives 8:28p (via DCA)

25k miles + $80 for close-in fee and taxes

3) If you have UA CC or status to access XN space, you can do:

IAH - LGA (Departs IAH at 8:20am, Arrives 12:47pm)

LGA - IAH (Departs IAH at 1:35pm, Arrives 4:47pm)

Same price of 25k miles. Both are booking into XN though so must have XN access to book those. Of course there is a chance that those will open up to X over the next day or two if they aren't sold but no guarantees.