I was there in November for a few days and spent a fair while researching hotels, The Foss hotel Baron is a 3* and is well priced (about $100 a night) and nice and central. I stayed in a very cheap hotel for a couple of nights (the Metropolitan) and then an expensive hotel (hotel Borg). The strange thing was the cheap hotel gave me a double bed -which just fit in the room, and the twin room in the expensive hotel was a tiny single! -nice hotel though.
I think in April you might be able to go whale watching, which I didnt get to do, though is highly reccomended.
I did the Golden Circle tour, which is good, though the only really interesting part is the Geysir so if you're pushed for time just do that. Everything is really expensive in Iceland, but the food options at Geysir are great, there is a cheap oprion which most people go for, but don't the 'expensive' option is for once pretty damn reasonable (and you get 10% off with your coach ticket) and is a great buffet with salmon etc. As soon as they drop you off for the Geysir go there and grab a table where you can watch the Geysir while you eat.
For the Blue Lagoon (which I agree you have to do) you can get a tour on your way back to the airport, which will pick you up from your hotel and take you to the lagoon (with your luggage), let you soak there for a while (remember to smear the mud all over your face!), and then take you to the airport, its all well choreographed so you wont have any problems. Make sure you leave time to eat in the good restaurant at the lagoon which really is excellent.
We also did the northern lights tour, which was way too cold to appreciate while we were there, though I have no idea what it will be like in April.
If you have time also go the
Lauger spa in Reykjavik, which has lots of different saunas and plunge pools in it, great fun.
I used
http://www.icelandexcursions.is/ to see what tours there were, but when I got there I ended up booking through someone else with the tourist office. Though I notice at one end (the main end) of the Laugavegur (the main road) I noticed an outpost of a tour company offering deals on tour bundles, so I'd check that out first.