ON the first afternoon, a drive to Akaka Falls and a visit to Laupahoehoe tsunami memorial (some of the most incredible views imaginable; nice park, rocky shores, crashing waves, cliffs) along the coast could take your afternoon, followed, given the time, by a quick visit to the waterfalls in Hilo.
Get an early start to the next day,and you could see the crater (in the park), walk through the Thurston Lava Tube, even drive Chain of Craters Road to the sea and see the sea arch and great vistas and interesting lava flows with hours to spare. Check with the U.S.G.S., but I don't think that you'll be able to see any flowing lava right now, and if it is, it usually requires a hike from outside the park.(http://hvo.wr.usgs.gov/activity/kilaueastatus.php; go to "recent observations")
If you have time and inclination, it's a fairly short drive to the lovely Lava Tree State Park (lush, easy stroll to view neat stuff near entrance: http://www.hawaiiweb.com/hawaii/lava-tree-state-park-hawaii-the-big-island.html), and/or the Kapoho tide pools (if you have snorkeling gear, it's pretty great). If you don't want to go in that direction, there are two botanic gardens near Hilo (one is on a gorgeous scenic drive).
Anyway, that's what I'd do, if I only had 1 1/2 days or so near Hilo.

I have been staying in Hilo for the past week. Right now it is pouring rain. We took Poke A Stick Lava Tour Sunday night to see the lava flowing (poked a stick in the lava) and watched it fall into the sea. Quite a sight! It required almost 2 hour walk over lava to get to the sites. Walk was outside the boundaries of the national park. Poured rain on the hike back.

Visited the national park on Monday. As it was raining we saw no views and did not hike ( it was OK as we have visited the park four other times). We ended up going to the Volcano Winery. Yesterday was partly cloudy with rain. Tuesday we had to go over to the Leeward side of the island to get sun.

With only a day and a half, hope the weather cooperates. Bring gear for rain as Hilo area is the rainiest part of the island.

Unless you are a long distance hiker over rugged terrain, the only way to see the lava is by helicopter tour. They leave from Hilo airport. I did this a few weeks ago and it was spectacular. However it must be a good weather day and that side of the island has had very stormy weather for the past week. Check the weather report before you go. If you drive to the top of the volcano you can get a stunning view of the red glowing gases coming out of the caldera but you cannot see the lava from there. In my experience it took one full day to see all the interesting sights in Volcano National Park.

Hi everyone. Thank you so much for all the information! It's certainly something I will take into account. Actually I will arrive tomorrow at Kona before lunch, and not Hilo. Planning to drive on the north and then check the sightings on the east side, north of Hilo (luapahoehoe and akaka). Next day I am keeping fully for volcanoes. Not entirely sure how to explore yet though. Poke a stick tours have morning hikes that end at noon, and I think although I am fit enough to take it. That would leave the afternoon for the park, plus a couple of hours on the next morning, before going to the airport. Does this plan make sense or is there something important I am not considering?

Finally, do you guys recommend any places for food. I am a food junkie and would really like to explore local cuisine and sushi, should there be nice places.

Once again, thank you for all your kind advice!

Terrific restaurant called the Hilo Bay Cafe; local produce, organic, nice ambiance, pretty reasonable prices, etc. http://www.hilobaycafe.com/
It's actually tucked into a shopping mall near the road to/from the volcano.

p.s. I would spend the day in the park, and eschew any tours, hiking or other.

We enjoyed our drive through the national park as much as anything we did on the Big Island. It was time well spent.

we have been considering that. We had experience driving through national parks in California and it was quite spectacular, so we were considering doing the same in Hawaii (after Hilo, it will be back to Oahu for a few more days). Our only question was whether we could see any flowing lava, which in a way we imagine will make a big difference to the whole experience. Are there spots in the natural park such as a scenic route, where one can spot lava flow (if the activity is high enough) even if from distance? I think if that's possible it would be a deal closer for us and we would stick to the park without excursions.

PS - Will be sure to check the Hilo Bay Cafe!

No, there won't be flowing lava visible this time unless you're in a helicopter (and sounds as though that will be a no-go due to time and visibility); however, the thermal experience (vents, smoking crater, heat, smell) in the park is still pretty awesome.

If you want a fun hike - and also - experience some of the volcanic venting (hissing) - a mile or so from the Visitor's Center - is the little Kilauea Iki. You walk through part of a rain forest before you descend down to the crater floor - and then it's a mile or so across. And no - it hasn't erupted since 1959 - but as the vents hiss - you do pick up your step a bit.

Here are some pics: http://tinyurl.com/by33akb

Of course, the OP stated "I am not a great hiker"; that hike is strenuous and would consume time.

You don't need to do any serious hiking to witness the steam vents. You can almost drive up to them!
http://www.nps.gov/havo/planyourvisit/craterrimtour_steam.htm

From the steam vents, you can walk a trail that is easy up to the Jaggar museum.

We drove from Hilo all the way to Jaggar one night, just so we could see the glow at night from the crater. By the time we got there, it was so misty, foggy etc.cthat nothing was visible. A long ride for not much...

http://www.nps.gov/havo/planyourvisit/index.htm