Depending on how long you have, you could always think about connecting flights. Belize is lovely at that time of year, and as there are no direct flights there from the UK so you'd have to change in any case, you could consider using your voucher to get to the US and then booking separate flights on to Belize. We're doing this for a trip to Guatemala as there are no direct flights there either - 2-4-1 flights in F to DFW and then separate onward flights. The same holds true for the Caribbean in terms of separate flights (although of course there are direct flights there so less of an incentive to connect!).

Or going to Asia, I agree that Thailand is great at that time too, and personally find Songkran a lot of fun (as long as you're prepared!). Or a reward flight to BKK/KUL/HKG/SIN followed by separate flights to Vietnam/Cambodia etc would work if you wanted to avoid Songkran in Thailand.

Alternatively, fly to Mumbai/Delhi and onward flights to Goa/Kerala/other Indian beaches.

You will need to bear in mind that these are all separate flights NOT connections and all the usual rules apply - you can't combine cash and reward bookings.