The iconic wildlife in the USA and northern Europe (bears, wolves, bison etc) is going to be hard to see at Easter. I wholeheartedly recommend the Grand Teton & Yellowstone area for wildlife but it's a May-October destination. Similarly, the Utah / Arizona / California loops will still be very cold / snowy at Easter and so you won't see much wildlife. Amazing scenery and geology, but not wildlife at that time of year.
+1 for Sabah / Sarawak, paying for a domestic from KL to KK or similar. Haven't been in many years though, so the rampant palm oil plantations are probably much worse than what we saw, and I cried. Japan also has some amazing wildlife, including bears, wild cats and 'snow monkeys' (though they probably won't be in the hot springs by April) but I think that would take more research / organising than Costa Rica!
India is a very good shout, because of the time of year. But you'll need to put in some effort to organise a reputable tour or way of getting around.
Almost impossible to do more than half way there with the companion voucher, but for the most wildlife density in any place I have ever visited, I would recommend Tasmania combined with either Kangaroo Island and the Grampians or Flinders Ranges (to see koalas, goanna, emus, more kangaroos) or north Queensland (for some tropical birds and mammals). With the right planning, TAS can almost guarantee (eastern grey) kangaroos, many smaller macropods (wallabies, pademelon, potoroo, bettong, bandicoots), echidna, wombats and platypus, and marine life including penguins. You might also see wild quoll and Tas devils if you stay in the right places. But TAS doesn't have koala or many really colourful parrots (they have a few but not the same at the tropical north).
Or much closer to home, there are still wild areas of Europe; you could easily spend a week focusing on seeing lynx in Spain or brown bears in Italy, Croatia or Greece, or walking the woodlands of eastern Poland (the boreal forest further north like Finland will still be snowy in April).
I personally would go for Kruger NP because it is so easy to organise and it is the most affordable way to 'do' a safari of iconic African wildlife. However, Easter is pretty much booked up in the southern half of the Park, so you would have to work around that at this late stage of booking. One downside is the heat, even in April you can expect ~30°C in the day in the Kruger area so air conditioning is helpful! Fly into Durban and out of Johannesburg and spend some time in KwaZulu-Natal (Drakensburgs if you like mountains, St Lucia / wetlands for relaxing beaches and hippos, and rhino at Hhluhluwe-iMfolozi) then through eSwatini to Kruger for a week. You'll be recommended anti-malarials for Kruger in April (end of the wet season) but no other vaccinations are needed. Or if you can afford it, the Okavango Delta area of Botswana is the world class area to visit, but it will still be hot, it can be the tail end of the wet season and the influx of water from Angola generally won't have started. But because of that many lodges do have specials on. Still insanely expensive mind.