I'm afraid that my visits were a little later in the year, so not entirely applicable in terms of migrants (and in April I was still too early for some of them including Roller, Golden Oriole and White Rumped Swift) but hopefully I can give you something of use.
Depending on how long you've got, I would recommend
El Pardo as a good place to start (not to be confused with the Gallery Museo Del Prado, which is worth a visit if that's your thing) - it's only a 20 minute cheap bus journey from the center of Madrid, but should hopefully set you up with a few of these big raptors and a good cross section of the commoner birds to save you from having to pay them as much stead when you head out elsewhere. The headliner is Spanish Imperial Eagle which I saw pretty quickly after crossing over the river at the one bridge and just carrying on until the road ran out. Eurasian Griffon and Eurasian Black Vulture should also be around as well as Black and Red Kite for your raptors. If you then head back down the hill and follow the river North as far as you feel like, that should give you decent opportunity to rack up both river and more general species, herons, tits, finches, short-toed treecreeper, eurasian nuthatch, hoopoe etc.
Bustards are very definitely around Madrid itself but would probably recommend finding someone who knows the specifics of where to look. The first trip I had a paid for excursion which netted us very close views of Little Bustard and a very distant view of Great Bustard, which was apparently very bad luck in that regard in addition to I think 12 raptor species through the morning (Griffon+Black Vulture, Hen, Marsh, Montagu's Harriers, Buzzard, Common+Lesser Kestrel, Booted+Golden Eagles, Black+Red Kites). The second trip was
In terms of Daimel, I was considering the same option but was suggested Laguna Navaseca instead of the main body of Daimiel, which I was very pleased with. It's a small body of water but when I was there at least was stuffed full of Flamingo, White-headed Duck, Black-necked Grebe, Black-winged Stilt and a great selection of more typical european waterbirds. Also had sightings of Spanish Sparrow, Purple Heron and Purple Swamphen, which were all then lifers for me. In terms of Owls, there were also a pair of little owls roosting on the roof of the abandoned barn to the East of the Road leading to the carpark from the South.
As for Monfrague - the park itself is pretty much a single road going through it so there is not much option in the way of going through it, but there are several points you can stop that overlook different points of interest. I have a feeling you may be unfortunately a little early to get the best of it but Black and Griffon vultures should be about I believe and some of the eagles (specifically Golden, Bonelli's and Spanish Imperial) should be around, though need a bit of patience. Unfortunately, I think you may be too early for Egyptian Vultures and Black Stork, though someone may be able to correct me on that.
If you are doing Monfrague, it may also be worth stopping off at Arrocampo on the way back - it's more or less on the way and it has I think the complete set of European Herons (not sure about Great Bittern) plus Black Winged Kites, better opportunities for Purple Swamphen and various other wetland birds.
